Absorbent foams made from high internal phase emulsions useful for acquiring aqueous fluids

ABSTRACT

Absorbent foam materials that are capable of acquiring and distributing aqueous fluids, especially discharged body fluids such as urine. These absorbent foams combine relatively high capillary absorption pressures and capacity-per-weight properties that allow them to acquire fluid, with or without the aid of gravity. These absorbent foams also give up this fluid efficiently to higher absorption pressure storage materials, including foam-based absorbent fluid storage components, without collapsing. These absorbent foams are made by polymerizing high internal phase emulsions (HIPEs).

This is a division of application Ser. No. 08/583,356, filed on Jan. 5,1996 now U.S. Pat. No. 5,571,849, which is a division of applicationSer. No. 08/520,793, filed on Aug. 30, 1995 now U.S. Pat. No. 5,550,167.

TECHNICAL FIELD OF THE INVENTION

This application relates to flexible, microporous, open-celled absorbentpolymeric foam materials. This application particularly relates toabsorbent foam materials made from high internal phase emulsions thatare capable of acquiring and distributing aqueous fluids, e.g., urine.

BACKGROUND OF THE INVENTION

The development of highly absorbent articles for use as disposablediapers, adult incontinence pads and briefs, and catamenial productssuch as sanitary napkins is the subject of substantial commercialinterest. The ability to provide high performance absorbent articlessuch as diapers has been contingent on the ability to develop relativelyabsorbent cores or structures that can acquire, distribute and storelarge quantities of discharged body fluids, in particular urine. In thisregard, the use of certain particulate absorbent polymers often referredto as "hydrogels," "superabsorbents" or "hydrocolloid" materials hasbeen particularly important. See, for example, U.S. Pat. No. 3,699,103(Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731(Harmon), issued Jun. 20, 1972, that disclose the use of suchparticulate absorbent polymers in absorbent articles. Indeed, thedevelopment of high performance diapers has been the direct consequenceof thinner absorbent cores that take advantage of the ability of theseparticulate absorbent polymers to absorb large quantities of dischargedaqueous body fluids, typically when used in combination with a fibrousmatrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al),issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issuedJun. 19, 1990, that disclose dual-layer core structures comprising afibrous matrix and particulate absorbent polymers useful in fashioninghigh performance diapers.

These particulate absorbent polymers have previously been unsurpassed intheir ability to retain large volumes of fluids, such as urine. Arepresentative example of such particulate absorbent polymers arelightly crosslinked polyacrylates. Like many of the other absorbentpolymers, these lightly crosslinked polyacrylates comprise amultiplicity of anionic (charged) carboxy groups attached to the polymerbackbone. It is these charged carboxy groups that enable the polymer toabsorb aqueous body fluids as the result of osmotic forces.

Absorbency based on capillary forces is also important in many absorbentarticles, including diapers. Capillary absorbents can offer superiorperformance in terms of the rate of fluid acquisition and wicking, i.e.the ability to move aqueous fluid away from the point of initialcontact. Indeed, the dual-layer core absorbent structures noted aboveuse the fibrous matrix as the primary capillary transport vehicle tomove the initially acquired aqueous body fluid throughout the absorbentcore so that it can be absorbed and retained by the particulateabsorbent polymer positioned in layers or zones of the core.

Other absorbent materials capable of providing capillary fluid transportare open-celled polymeric foams. Indeed, certain types of polymericfoams have been used in absorbent articles for the purpose of actuallyimbibing, wicking and/or retaining aqueous body fluids. See, forexample, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971(absorbent pad for diapers and the like where the primary absorbent is ahydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi),issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can beused in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garveyet al), issued Apr. 26, 1988 (absorbent composite structure such asdiapers, feminine care products and the like that contain spongeabsorbents made from certain types of super-wicking, crosslinkedpolyurethane foams).

If made appropriately, open-celled hydrophilic polymeric foams canprovide features of capillary fluid acquisition, transport and storagerequired for use in high performance absorbent cores. Absorbent articlescontaining such foams can possess desirable wet integrity, can providesuitable fit throughout the entire period the article is worn, and canminimize changes in shape during use (e.g., uncontrolled swelling,bunching). In addition, absorbent articles containing such foamstructures can be easier to manufacture on a commercial scale. Forexample, absorbent diaper cores can simply be stamped out fromcontinuous foam sheets and can be designed to have considerably greaterintegrity and uniformity than absorbent fibrous webs. Such foams canalso be prepared in any desired shape, or even formed into single-piecediapers.

Particularly suitable absorbent foams for absorbent products such asdiapers have been made from High Internal Phase Emulsions (hereafterreferred to as "HIPE"). See, for example, U.S. Pat. No. 5,260,345(DesMarais et al), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224(DesMarais et al), issued Dec. 7, 1993. These absorbent HIPE foamsprovide desirable fluid handling properties, including: (a) relativelygood wicking and fluid distribution characteristics to transport theimbibed urine or other body fluid away from the initial impingement zoneand into the unused balance of the foam structure to allow forsubsequent gushes of fluid to be accommodated; and (b) a relatively highstorage capacity with a relatively high fluid capacity under load, i.e.under compressive forces. These HIPE absorbent foams are alsosufficiently flexible and soft so as to provide a high degree of comfortto the wearer of the absorbent article; some can be made relatively thinuntil subsequently wetted by the absorbed body fluid. See also U.S. Pat.No. 5,147,345 (Young et al), issued Sep. 15, 1992 and U.S. Pat. No.5,318,554 (Young et al), issued Jun. 7, 1994, which discloses absorbentcores having a fluid acquisition/distribution component that can be ahydrophilic, flexible, open-celled foam such as a melamine-formaldehydefoam (e.g., BASOTECT made by BASF), and a fluid storage/redistributioncomponent that is a HIPE-based absorbent foam.

These foam-based acquisition/distribution components should allow rapidfluid acquisition, as well as efficient partitioning or distribution offluid to other components of the absorbent core having higher absorptionpressures than the desorption pressure of the acquisition/distributionfoam. This property of fluid desorption to other core components isimportant in providing the ability to accept repeated discharges orloadings of fluid and to maintain the skin dryness of the wearer. Italso allows the acquisition/distribution foam to serve as a void volumereservoir, or buffer zone, to temporarily hold fluid that can beexpressed from the storage components of the core when extraordinarilyhigh pressures are encountered during use of the absorbent article.

In giving this fluid to other core components, these foam-basedacquisition/distribution components should do so without densifying orcollapsing. Foam-based acquisition/distribution components should alsoreadily accept fluid, with or without the aid of gravity. Foam-basedacquisition/distribution components should further provide goodaesthetics, be soft and resilient in structure, and have good physicalintegrity in both wet and dry states.

Materials useful as such acquisition/distribution components aredescribed and claimed in co-pending U.S. Ser. No. 08/370,695, filed Jan.10, 1995, by Stone, et al. While the materials described thereinfunction well as such a component, there remains a desire to improvetheir ability to desorb fluid into a storage component. Accordingly, itwould be desirable to be able to make an open-celled absorbent polymericfoam material, in particular an absorbent HIPE foam, that: (1) canfunction as an acquisition/distribution component in an absorbent core;(2) has improved desorption properties to allow other core componentshaving higher absorption pressures than the desorption pressure of theacquisition/distribution foam to partition away fluid without theacquisition/distribution foam collapsing; (3) keeps the wearer's skindry, even in "gush" situations and even when subjected to compressiveload; (4) is soft, flexible and comfortable to the wearer of theabsorbent article; and (5) has a relatively high capacity for fluid soas to provide diapers and other absorbent articles that efficientlyutilize core components.

DISCLOSURE OF THE INVENTION

The present invention relates to polymeric foam materials that arecapable of acquiring and distributing aqueous fluids, especiallydischarged body fluids such as urine. These absorbent polymeric foammaterials comprise a hydrophilic, flexible, nonionic polymeric foamstructure of interconnected open-cells.

The foam structure has:

A) a capacity after desorption at 30 cm of less than about 10% of thefoam's free absorbent capacity;

B) a capillary absorption pressure (i.e., height at 50% capacity) offrom about 3 to about 20 cm;

C) a capillary desorption pressure (i.e., height at 50% capacity) offrom about 8 to about 25 cm;

D) a resistance to compression deflection of from about 5 to about 85%when measured under a confining pressure of 0.74 psi; and

E) a free absorbent capacity of from about 12 to about 125 g/g.

Besides acquiring and distributing body fluids rapidly, the absorbentfoams of the present invention give up this fluid efficiently to otherfluid storage components, including foam-based fluid storage components.The absorbent foams of the present invention combine relatively highcapillary absorption pressures and capacity-per-weight properties(compared to conventional foams) that allow them to acquire fluid, withor without the aid of gravity. While not being bound by theory, it isbelieved the improved desorption properties of the present foams is due,at least in part, to a more uniform distribution of cell sizes comparedto prior foams, including those described in co-pending U.S. Ser. No.08/370,695, filed Jan. 10, 1995 (Stone et al.). In any event, theenhanced desorption properties are obtainable because of the processingimprovements discussed in detail below.

The absorbent foams of the present invention also provide goodaesthetics due to their soft, resilient structure and physicalintegrity. As a result, the absorbent foams of the present invention areparticularly attractive for high performance absorbent articles such asdiapers, adult incontinence pads or briefs, sanitary napkins, and thelike.

A particularly important attribute of the foams of the present inventionis that they do not collapse when desorbed by other components in theabsorbent core. While not being bound by theory, it is believed thatthis resistance to compression (i.e., resistance to collapse) byhydrostatic pressures is due to the desorption pressure of these foamsin their expanded state being less than the pressure required forcompression deflection. A related important attribute is that thesefoams, when wetted, spontaneously reexpand after application and releaseof mechanical compression, even if the foams do not reabsorb fluid. Thismeans these foams imbibe air when dewatered by either desorption, bymechanical compression, or a combination thereof, when expanded or whenreturning to an expanded state. As a result, the capability of thesefoams to quickly acquire fluids is restored and the foam is able toprovide a drier feel.

The present invention further relates to a process for obtaining theseabsorbent foams by polymerizing a specific type of water-in-oil emulsionor HIPE having a relatively small amount of an oil phase and arelatively greater amount of a water phase. This process comprises thesteps of:

A) forming a water-in-oil emulsion at a temperature of about 50° C. orhigher, under low shear mixing at less than about 1000 sec⁻¹, from:

1) an oil phase comprising:

a) from about 85 to about 98% by weight of a monomer component capableof forming a copolymer having a Tg of about 35° C. or lower, the monomercomponent comprising:

i) from about 30 to about 80% by weight of at least one substantiallywater-insoluble monofunctional monomer capable of forming an atacticamorphous polymer having a Tg of about 25° C. or lower;

ii) from about 5 to about 40% by weight of at least one substantiallywater-insoluble monofunctional comonomer capable of imparting toughnessabout equivalent to that provided by styrene;

iii) from about 5 to about 25% by weight of a first substantiallywater-insoluble, polyfunctional crosslinking agent selected from divinylbenzenes, trivinylbenzenes, divinyltoluenes, divinylxylenes,divinylnaphthalenes divinylalkylbenzenes, divinylphenanthrenes,divinylbiphenyls, divinyldiphenylmethanes, divinylbenzyls,divinylphenylethers, divinyldiphenylsulfides, divinylfurans,divinylsulfide, divinyl sulfone, and mixtures thereof; and

iv) from 0 to about 15% by weight of a second substantiallywater-insoluble, polyfunctional crosslinking agent selected frompolyfunctional acrylates, methacrylates, acrylamides, methacrylamides,and mixtures thereof; and

b) from about 2 to about 15% by weight of an emulsifier component whichis soluble in the oil phase and which is suitable for forming a stablewater-in-oil emulsion, the emulsion component comprising: (i) a primaryemulsifier having at least about 40% by weight emulsifying componentsselected from diglycerol monoesters of linear unsaturated C₁₆ -C₂₂ fattyacids, diglycerol monoesters of branched C₁₆ -C₂₄ fatty acids,diglycerol monoaliphatic ethers of branched C₁₆ -C₂₄ alcohols,diglycerol monoaliphatic ethers of linear unsaturated C₁₆ -C₂₂ fattyalcohols, diglycerol monoaliphatic ethers of linear saturated C₁₂ -C₁₄alcohols, sorbitan monoesters of linear unsaturated C₁₆ -C₂₂ fattyacids, sorbitan monoesters of branched C₁₆ -C₂₄ fatty acids, andmixtures thereof; or (ii) the combination a primary emulsifier having atleast 20% by weight of these emulsifying components and certainsecondary emulsifiers in a weight ratio of primary to secondaryemulsifier of from about 50:1 to about 1:4; and

2) a water phase comprising an aqueous solution containing: (i) fromabout 0.2 to about 20% by weight of a water-soluble electrolyte; and(ii) an effective amount of a polymerization initiator;

3) a volume to weight ratio of water phase to oil phase in the range offrom about 12:1 to about 125:1; and

B) polymerizing the monomer component in the oil phase of thewater-in-oil emulsion to form a polymeric foam material; and

C) optionally dewatering the polymeric foam material.

The process of the present invention allows the formation of theseabsorbent foams that are capable of acquiring, distributing, and rapidlydesorbing fluids as a result of a combination of two factors, both ofwhich are discussed in detail below. In general, one factor is the useof low shear mixing during HIPE formation. The other is the use of morerobust emulsifier systems that allow the HIPE to be formed and poured atrelatively high temperatures, e.g. about 50° C. or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a graphical plot of the absorption curves ofthree HIPE foams poured at different pin to wall gaps and pin tipspeeds.

FIG. 2 of the drawings is a graphical plot of the desorption curves ofthese same three HIPE foams.

FIG. 3 of the drawings is a photomicrograph (50×magnification) of asection of a representative absorbent polymeric foam according to thepresent invention made from HIPE having a 40:1 water-to-oil weight ratioand poured at 147° C., and where the monomer component consisted of a42:58 weight ratio of divinyl benzene (DVB, 42.9%divinylbenzene):2-ethylhexyl acrylate and where 6% (by weight of the oilphase) of diglycerol monooleate (DGMO) emulsifier was used.

FIG. 4 of the drawings is a photomicrograph (250×magnification) of thefoam of FIG. 3.

FIGS. 5 and 6 of the drawings represent, respectively, a top view and aside view of a multi-layer core configuration where the fluidstorage/redistribution component overlies a subjacent fluidacquisition/distribution component.

FIG. 7. of the drawings represents a blown-apart view of the componentsof a diaper structure also of multi-layer core configuration having anhourglass-shaped fluid acquisition/distribution foam layer overlying afluid storage/redistribution layer with a modified hourglass shape.

DETAILED DESCRIPTION OF THE INVENTION

I. Polymeric Absorbent Foams

A. General Foam Characteristics

Polymeric foams according to the present invention useful in absorbentarticles and structures are those which are relatively open-celled. Thismeans the individual cells of the foam are in complete, unobstructedcommunication with adjoining cells. The cells in such substantiallyopen-celled foam structures have intercellular openings or "windows"that are large enough to permit ready fluid transfer from one cell tothe other within the foam structure.

These substantially open-celled foam structures will generally have areticulated character with the individual cells being defined by aplurality of mutually connected, three dimensionally branched webs. Thestrands of polymeric material making up these branched webs can bereferred to as "struts." Open-celled foams having a typical strut-typestructure are shown by way of example in the photomicrographs of FIGS. 3and 4. For purposes of the present invention, a foam material is"open-celled" if at least 80% of the cells in the foam structure thatare at least 1 μm in size are in fluid communication with at least oneadjacent cell.

In addition to being open-celled, these polymeric foams are sufficientlyhydrophilic to permit the foam to absorb aqueous fluids. The internalsurfaces of the foam structures are rendered hydrophilic by residualhydrophilizing surfactants left in the foam structure afterpolymerization, or by selected post-polymerization foam to treatmentprocedures, as described hereafter.

The extent to which these polymeric foams are "hydrophilic" can bequantified by the "adhesion tension" value exhibited when in contactwith an absorbable test liquid. The adhesion tension exhibited by thesefoams can be determined experimentally using a procedure where weightuptake of a test liquid, e.g., synthetic urine, is measured for a sampleof known dimensions and capillary suction specific surface area. Such aprocedure is described in greater detail in the TEST METHODS section ofU.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7, 1995, which isincorporated by reference. Foams which are useful as absorbents in thepresent invention are generally those which exhibit an adhesion tensionvalue of from about 15 to about 65 dynes/cm, more preferably from about20 to about 65 dynes/cm, as determined by capillary suction uptake ofsynthetic urine having a surface tension of 65±5 dynes/cm.

An important aspect of these foams is their glass transition temperature(Tg). The Tg represents the midpoint of the transition between theglassy and rubbery states of the polymer. Foams that have a higher Tgthan the temperature of use can be very strong but will also be veryrigid and potentially prone to fracture. Such foams also typically takea long time to respond when used at temperatures colder than the Tg ofthe polymer. The desired combination of mechanical properties,specifically strength and resilience, typically necessitates a fairlyselective range of monomer types and levels to achieve these desiredproperties.

For foams of the present invention, the Tg should be as low as possible,so long as the foam has acceptable strength. Accordingly, monomers areselected as much as possible that provide corresponding homopolymershaving lower Tg's. It has been found that the chain length of the alkylgroup on the acrylate and methacrylate comonomers can be longer thanwould be predicted from the Tg of the homologous homopolymer series.Specifically, it has been found that the homologous series of alkylacrylate or methacrylate homopolymers have a minimum Tg at a chainlength of 8 carbon atoms. By contrast, the minimum Tg of the copolymersof the present invention occurs at a chain length of about 12 carbonatoms. (While the alkyl substituted styrene monomers can be used inplace of the alkyl acrylates and methacrylates, their availability iscurrently extremely limited).

The shape of the glass transition region of the polymer can also beimportant, i.e., whether it is narrow or broad as a function oftemperature. This glass transition region shape is particularly relevantwhere the in-use temperature (usually ambient or body temperature) ofthe polymer is at or near the Tg. For example, a broader transitionregion can mean an incomplete transition at in-use temperatures.Typically, if the transition is incomplete at the in-use temperature,the polymer will evidence greater rigidity and will be less resilient.Conversely, if the transition is completed at the in-use temperature,then the polymer will exhibit faster recovery from compression.Accordingly, it is desirable to control the Tg and the breadth of thetransition region of the polymer to achieve the desired mechanicalproperties. Generally, it is preferred that the Tg of the polymer be atleast about 10° C. lower than the in-use temperature. (The Tg and thewidth of the transition region are derived from the loss tangent vs.temperature curve from a dynamic mechanical analysis (DMA) measurement,as described in the Test Methods section hereafter).

B. Foam Characteristics Important to Acquiring and Distributing AqueousFluids Without Collapsing

1. Capillary. Absorption and Desorption Pressures

An important property of useful absorbent foams according to the presentinvention is their capillary absorption pressure. Capillary absorptionpressure refers to the ability of the foam to wick fluid vertically. SeeP. K. Chatterjee and H. V. Nguyen in "Absorbency," Textile Science andTechnology, Vol. 7; P. K. Chatterjee, Ed.; Elsevier: Amsterdam, 1985;Chapter 2.! For the purposes of the present invention, the capillaryabsorption pressure of interest is the hydrostatic head at which thevertically wicked fluid loading is 50% of the free absorbent capacityunder equilibrium conditions at 31° C. The hydrostatic head isrepresented by a column of fluid (e.g., synthetic urine) of height h. Asillustrated in FIG. 1, for foams of the present invention, this istypically the inflection point on the capillary absorption curve.

FIG. 1 depicts the absorption curves for three foams. The absorptionpressures were determined from these absorption curves and aresummarized in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Foam       Shear Rate Absorption Pressure                                     ______________________________________                                        A          2767 sec.sup.-1 *                                                                        >8                                                      B           540 sec.sup.-1 **                                                                       9                                                       C           180 sec.sup.-1 ***                                                                      6                                                       ______________________________________                                         *: 0.030 in. (0.8 mm) (1.5% of Cylinder Diameter) pin wall to gap; 83         in/sec (212 cm/sec) pin tip speed.                                            **: 0.060 in. (1.5 mm) (3% of Cylinder Diameter) pin wall to gap; 32.4        in/sec (83.3 cm/sec) pin tip speed.                                           ***: 0.120 in. (3 mm) (6% of Cylinder Diameter) pin wall to gap; 21.6         in/sec (54.9 cm/sec) pin tip speed.                                      

Of particular importance to the ability of the absorbent foams of thepresent invention to function as useful fluid acquisition anddistribution components is their capillary desorption pressure.Capillary desorption pressure refers to the foam's ability to hold ontofluid at various hydrostatic heads at equilibrium conditions at 22° C.For the purposes of the present invention, the capillary desorptionpressure of interest is the hydrostatic head (i.e., height) at which thefluid loading is 50% of the free absorbent capacity under equilibriumconditions at 22° C. As illustrated in FIG. 2, for foams of the presentinvention, this is typically the inflection point on the capillarydesorption curve.

FIG. 2 depicts the desorption curves of the same three foams. Thedesorption pressures were determined from these desorption curves andare summarized in Table 2 below:

                  TABLE 2                                                         ______________________________________                                        Foam Shear Rate Desorption Pressure                                                                        Capacity After Desorption.sup.1                  ______________________________________                                        A    2767 sec.sup.-1 *                                                                        27            38%                                             B     540 sec.sup.-1 **                                                                       19           5.8%                                             C     180 sec.sup.-1 ***                                                                      16           3.3%                                             ______________________________________                                         *: 0.030 in. (0.8 mm) (1.5% of Cylinder Diameter) pin wall to gap; 83         in/sec (212 cm/sec) pin tip speed.                                            **: 0.060 in. (1.5 mm) (3% of Cylinder Diameter) pin wall to gap; 32.4        in/sec (83.3 cm/sec) pin tip speed.                                           ***: 0.120 in. (3 mm) (6% of Cylinder Diameter) pin wall to gap; 21.6         in/sec (54.9 cm/sec) pin tip speed.                                           .sup.1 : measured from desorption curve as percent capacity at 30 cm (g/g     compared with free absorbent capacity (g/g).                             

The capillary desorption pressure is important relative to theabsorption pressure of other absorbent components, especially thoseintended for fluid storage. If the fluid acquisition component of theabsorbent article holds the acquired fluid too tenaciously, this willinhibit the ability of these other components to partition fluid away.This can cause the acquisition component to remain so heavily loadedwith fluid that the absorbent article is more susceptible to leaking.The foams of the present invention have desorption pressures low enoughso that fluid storage components can effectively dry out (i.e. desorb)these foams. This restores the capacity of the foam to accept furtherfluid "gushes" (either from the wearer or from squeeze out from thestorage components) and keeps the layer (e.g., topsheet) next to theskin of the wearer comparatively dry. The data in Table 2 above showshow this property can be adjusted by selection of appropriate processingconditions (e.g., shear rate, which is dictated by impeller tip speedand pin to wall gap.). Importantly, foams B and C, which were formedunder the low shear conditions as discussed herein, have the requisitedesorption pressures and capacity after desorption at 30 cm. Incontrast, foam A, which was formed under higher shear, does not have therequisite desorption properties.

The absorbent foams of the present invention can be readily desorbed byother components of the absorbent core that store such fluids, includingthose comprising conventional absorbent gelling materials such as aredisclosed in, for example, U.S. Pat. No. 5,061,259 (Goldman et. al),issued Oct. 29, 1991, U.S. Pat. No. 4,654,039 (Brandt et al), issuedMar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Pat. No. Re. 32,649), U.S.Pat. No. 4,666,983 (Tsubakimoto et al), issued May 19, 1987, and U.S.Pat. No. 4,625,001 (Tsubakimoto et al), issued Nov. 25, 1986, all ofwhich are incorporated by reference; as well as absorbentmacrostructures made from these absorbent gelling materials such asthose disclosed in, for example, U.S. Pat. No. 5,102,597 (Roe et al),issued Apr. 7, 1992, and U.S. Pat. No. 5,324,561 (Rezai et al), issuedJun. 23, 1994, both of which are incorporated by reference). Indeed,these absorbent foams can be readily desorbed by other absorbentpolymeric foams that store the acquired fluid, such as those disclosedin, for example, U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec.7, 1993; U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7, 1995; andcopending U.S. application Ser. No. 08/370,922 (Thomas A. DesMarais etal), filed Jan. 10, 1995, Case No. 5541, all of which are incorporatedby reference. Accordingly, the absorbent foams of the present inventionfunction very well in multiple "gush" situations to move the acquiredfluid rapidly to other fluid storage components of the absorbentstructure.

Capillary absorption pressures can be measured using a vertical wickingabsorbent capacity test as described in greater detail in the TESTMETHODS section of U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7,1995, which is incorporated by reference, except at 31° C. rather than37° C. Data from the vertical wicking absorbent capacity test providesthe curve from which the capillary absorption pressure is determined.

Capillary desorption pressure can be measured using the proceduredescribed in the TEST METHODS section. To generate the data for adesorption curve, a foam sample is saturated with water, hung verticallyand then allowed to desorb until equilibrium is reached (referred toherein as "desorpton equilibrium"). The fluid loading is then plotted asa function of height. The capillary desorption pressure, i.e., thehydrostatic head at which the fluid loading is 50% of the free absorbentcapacity, is determined from this curve.

Suitable absorbent foams according to the present invention havecapillary absorption pressures of from about 3 to about 20 cm andcapillary desorption pressures of about 8 to about 25 cm. Particularlypreferred absorbent foams have capillary absorption pressures of fromabout 3 to about 15 cm and capillary desorption pressures of about 8 toabout 20 cm.

2. Capacity After Desorption

Another measure of a foam's ability desorb fluid is to determine thefluid capacity of different portions of the foam after desorption. Asused herein, the term "capacity after desorption" refers to the relativeamount of test fluid (synthetic urine) a given foam sample, or portionthereof, retains in its cellular structure per unit mass of solidmaterial in the sample, after reaching desorption equilibrium. The foamsof the present invention have a capacity after desorption, at 30 cm,(referred to herein as "capacity after desorption at 30 cm") of lessthan about 10% of the foam's free absorbent capacity. Capacity afterdesorption can be determined by referring to the desorption curves(discussed supra), such as those of FIG. 2; free absorbent capacity canbe determined as discussed below.

Preferably, the foams of the present invention will have a capacityafter desorption at 28 cm that is less than about 10% of the foam's freeabsorbent capacity.

3. Resistance to Compression Deflection

An important mechanical feature of the absorbent foams of the presentinvention is their strength as determined by their resistance tocompression deflection (RTCD). The RTCD exhibited by the foams herein isa function of the polymer modulus, as well as the density and structureof the foam network. The polymer modulus is, in turn, determined by: a)the polymer composition; b) the conditions under which the foam waspolymerized (for example, the completeness of polymerization obtained,specifically with respect to crosslinking); and c) the extent to whichthe polymer is plasticized by residual material, e.g., emulsifiers, leftin the foam structure after processing.

To be useful as fluid acquisition/distribution components in absorbentcores of absorbent articles such as diapers, the foams of the presentinvention must be suitably resistant to deformation or compression byforces encountered when such absorbent materials are engaged in theabsorption and retention of fluids. This is particularly important asfluids are partitioned, either due to a sorption pressure gradient orsqueeze out, from the acquisition/distribution components and into otherfluid storage components in the absorbent core. Indeed, theacquisition/distribution foams of the present invention provide abalance of capillary desorption pressure and foam strength to avoidundesirable collapse during partitioning.

If the capillary desorption pressure of the foam is greater than itsRTCD and/or its re-expansion strength (i.e., expansion pressure at aparticular % compression), it will tend to collapse upon desorption andthus leave the foam in a saturated, densified state. In this state, theacquisition/distribution foam can feel wet to the touch, leading towetter skin for the wearer. It would also impede the rate of acquiringadditional fluid gushes.

If the foams are too strong, however, they will look and feel stiff,leading to poor aesthetics. Also, one mechanism by which foams of thepresent invention can distribute and partition fluid involves mechanicalpumping. Thus it can be advantageous for the acquisition/distributionfoam to be squeezed to some degree by normal pressures experienced bythe wearer during use to promote this additional partitioning mechanism.

The RTCD exhibited by the polymeric foams of the present invention canbe quantified by determining the amount of strain produced in a sampleof saturated foam held under a certain confining pressure for aspecified period of time. The method for carrying out this particulartype of test is described hereafter in the TEST METHODS section. Foamsuseful as absorbents for acquiring and distributing fluids are thosewhich exhibit a resistance to compression deflection such that aconfining pressure of 0.74 psi (5.1 kPa) produces a strain of typicallyfrom about 5 to about 85% compression of the foam structure. Preferablythe strain produced under such conditions will be in the range fromabout 5 to about 65%, most preferably from about 5 to about 50%.

4. Recovery from Wet Compression

Recovery from wet compression (RFWC) relates to the tendency orpropensity of a piece of wet foam material to quickly return to itsoriginal dimensions after being deformed or compressed under forcesencountered in manufacture or use, without having a reservoir of freefluid to draw from during re-expansion. Many high capillary pressurefoams, such as those described in U.S. Pat. No. 5,268,224 (DesMarais etal.), issued Dec. 7, 1993, and in U.S. Pat. No. 5,387,207 (Dyer et al),issued Feb. 7, 1995, will not readily reexpand. It has also been foundthat re-expansion is even more difficult for an acquisition/distributionfoam when it is in competition for fluid with a higher sorption pressurecomponent, such as is typically encountered in absorbent cores.

A suitable procedure for determining recovery from wet compression isset forth in the TEST METHODS section. Such a procedure in generalinvolves compression of a foam sample that has previously been saturatedto its free absorbent capacity with synthetic urine while positioned ontop of a high capillary absorption pressure material. Samples aremaintained under a strain of 75% compression at a constant temperature(31° C.) for a period of five minutes, then are released fromcompression. After two minutes of competing for the fluid with thehigher sorption pressure material (the sample having had the opportunityto re-expand), the sample is separated and its thickness measured. Theextent to which the sample recovers its thickness is taken as a measureof the recovery from wet compression of the sample.

Preferred absorbent foams of the present invention will generallyexhibit a recovery to at least about 60% of the fully expanded thicknesswithin two minutes of being released from compression. More preferably,such preferred foam materials will have a recovery from wet compressionof at least about 75%, most preferably at least about 90%, of the fullyexpanded thickness within one minute of being released from compression.

5. Free Absorbent Capacity

Another important property of absorbent foams according to the presentinvention is their free absorbent capacity. "Free absorbent capacity" isthe total amount of test fluid (synthetic urine) which a given foamsample will absorb into its cellular structure per unit mass of solidmaterial in the sample. The foams which are especially useful inabsorbent articles such as diapers will at least meet a minimum freeabsorbent capacity. To be especially useful in absorbent articles forabsorbing urine, the absorbent foams of the present invention shouldhave a free capacity of from about 12 to about 125 g/g, preferably fromabout 20 to about 90 g/g, more preferably from about 25 to about 75 g/g,and most preferably from about 30 to about 55 g/g, of synthetic urineper gram of dry foam material. The procedure for determining the freeabsorbent capacity of the foam is described hereafter in the TESTMETHODS section.

C. Other Properties of Polymeric Foam

1. Cell and Hole Sizes

A feature that can be useful in defining preferred polymeric foams isthe cell structure. Foam cells, and especially cells that are formed bypolymerizing a monomer-containing oil phase that surrounds relativelymonomer-free water-phase droplets, will frequently be substantiallyspherical in shape. These spherical cells are connected to each other byopenings, which are referred to hereafter as holes between cells. Boththe size or "diameter" of such spherical cells and the diameter of theopenings (holes) between the cells are commonly used for characterizingfoams in general. Since the cells, and holes between the cells, in agiven sample of polymeric foam will not necessarily be of approximatelythe same size; average cell and hole sizes, i.e., average cell and holediameters, will often be specified.

Cell and hole sizes are parameters that can impact a number of importantmechanical and performance features of the foams according to thepresent invention, including the fluid wicking properties of thesefoams, as well as the capillary pressure that is developed within thefoam structure. A number of techniques are available for determining theaverage cell and hole sizes of foams. The most useful technique involvesa simple measurement based on the scanning electron photomicrograph of afoam sample. FIGS. 3 and 4, for example, show a typical HIPE foamstructure according to the present invention. Superimposed on thephotomicrograph of FIG. 4 is a scale representing a dimension of 20 μm.Such a scale can be used to determine average cell and hole sizes by animage analysis procedure. The foams useful as absorbents for aqueousfluids in accordance with the present invention will preferably have anumber average cell size of from about 20 to about 200 μm, morepreferably from about 30 to about 190 μm, and most preferably from about80 to about 180 μm; and a number average hole size of from about 5 toabout 45 μm, preferably from about 8 to about 40 μm, and most preferablyfrom about 20 to about 35 μm.

2. Capillary Suction Specific Area

"Capillary suction specific surface area" is a measure of thetest-liquid-accessible surface area of the polymeric network accessibleto the test fluid. Capillary suction specific surface area is determinedboth by the dimensions of the cellular units in the foam and by thedensity of the polymer, and is thus a way of quantifying the totalamount of solid surface provided by the foam network to the extent thatsuch a surface participates in absorbency.

For purposes of this invention, capillary suction specific surface areais determined by measuring the amount of capillary uptake of a lowsurface tension liquid (e.g., ethanol) which occurs within a foam sampleof a known mass and dimensions. A detailed description of such aprocedure for determining foam specific surface area via the capillarysuction method is set forth in the TEST METHODS section of U.S. Pat. No.5,387,207 (Dyer et al), issued Feb. 7, 1995. Any reasonable alternativemethod for determining capillary suction specific surface area can alsobe utilized.

The foams of the present invention useful as absorbents are those thathave a capillary suction specific surface area of at least about 0.2 m²/g. Typically, the capillary suction specific surface area is in therange from about 0.3 to about 5 m² /g, preferably from about 0.3 toabout 2.5 m² /g, most preferably from about 0.3 to about 1.5 m² /g.

3. Surface Area per Foam Volume

Specific surface area per foam volume can be useful for empiricallydefining foam structures that will not collapse, or remain in acollapsed state, when desorbed, e.g., dried or compressed while in a wetstate. See U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7, 1995,where specific area per foam volume is discussed in detail with regardto collapsed foams. As used herein, "specific surface area per foamvolume" refers to the capillary suction specific surface area of thefoam structure times its foam density in the expanded state. It has beenfound that certain maximum specific surface area per foam volume valuescorrelate with the ability of the foam structure to remain in anexpanded state when desorbed, or to quickly return to an expanded stateafter being compressed while in a wet state. Foams according to thepresent invention have specific surface area per foam volume values ofabout 0.06 m² /cc or less, preferably from about 0.0075 to about 0.06 m²/cc, more preferably from about 0.0075 to about 0.04 m² /cc, and mostpreferably from about 0.008 to about 0.02 m² /cc.

4. Foam Density

"Foam density" (i.e., in grams of foam per cubic centimeter of foamvolume in air) is specified herein on a dry basis. The density of thefoam, like capillary suction specific surface area, can influence anumber of performance and mechanical characteristics of absorbent foams.These include the absorbent capacity for aqueous fluids and thecompression deflection characteristics.

Any suitable gravimetric procedure that will provide a determination ofmass of solid foam material per unit volume of foam structure can beused to measure foam density. For example, an ASTM gravimetric proceduredescribed more fully in the TEST METHODS section of U.S. Pat. No.5,387,207 (Dyer et al) is one method that can be employed for densitydetermination. Polymeric foams of the present invention useful asabsorbents have dry basis density values in the range of from about0.0079 to about 0.077 g/cc, preferably from about 0.011 to about 0.038g/cc, and most preferably from about 0.015 to about 0.032 g/cc.

II. Preparation of Polymeric Foams From HIPE

A. In General

Polymeric foams according to the present invention can be prepared bypolymerization of certain water-in-oil emulsions having a relativelyhigh ratio of water phase to oil phase commonly known in the art as"HIPEs. Polymeric foam materials which result from the polymerization ofsuch emulsions are referred to hereafter as "HIPE foams."

The relative amounts of the water and oil phases used to form the HIPEsare, among many other parameters, important in determining thestructural, mechanical and performance properties of the resultingpolymeric foams. In particular, the ratio of water to oil in theemulsion varies inversely with ultimate foam density and can influencethe cell size and capillary suction specific surface area of the foamand dimensions of the struts that form the foam. The emulsions used toprepare the HIPE foams of this invention will generally have a volume toweight ratio of water phase to oil phase in the range of from about 12:1to about 125:1, and most typically from about 25:1 to about 90:1.Particularly preferred foams can be made from HIPEs having ratios offrom about 30:1 to about 65:1.

1. Oil Phase Components

The continuous oil phase of the HIPE comprises monomers that arepolymerized to form the solid foam structure. This monomer component isformulated to be capable of forming a copolymer having a Tg of about 35°C. or lower, and typically from about 15° to about 30° C. (The methodfor determining Tg by Dynamic Mechanical Analysis (DMA) is describedhereafter in the TEST METHODS section.) This monomer component includes:(a) at least one monofunctional monomer whose atactic amorphous polymerhas a Tg of about 25° C. or lower (see Brandup, J.; Immergut, E. H."Polymer Handbook", 2nd Ed., Wiley-Interscience, New York, N.Y., 1975,III-139.); (b) at least one monofunctional comonomer to improve thetoughness or tear resistance of the foam; (c) a first polyfunctionalcrosslinking agent; and (d) optionally a second polyfunctionalcrosslinking agent. Selection of particular types and amounts ofmonofunctional monomer(s) and comonomer(s) and polyfunctionalcross-linking agent(s) can be important to the realization of absorbentHIPE foams having the desired combination of structure, mechanical, andfluid handling properties which render such materials suitable for usein the invention herein.

The monomer component comprises one or more monomers that tend to impartrubber-like properties to the resulting polymeric foam structure. Suchmonomers can produce high molecular weight (greater than 10,000) atacticamorphous polymers having Tg's of about 25° C. or lower. Monomers ofthis type include, for example, the (C₄ -C₁₄) alkyl acrylates such asbutyl acrylate, hexyl acrylate, octyl acrylate, 2-ethylhexyl acrylate,nonyl acrylate, decyl acrylate, dodecyl (lauryl) acrylate, isodecylacrylate, tetradecyl acrylate, aryl acrylates and alkaryl acrylates suchas benzyl acrylate, nonylphenyl acrylate, the (C₆ -C₁₆) alkylmethacrylates such as hexyl methacrylate, octyl methacrylate, nonylmethacrylate, decyl methacrylate, isodecyl methacrylate, dodecyl(lauryl) methacrylate, tetradecyl methacrylate, (C₄ -C₁₂) alkyl styrenessuch as p-n-octylstyrene, acrylamides such as N-octadecyl acrylamide,isoprene, butadiene, and combinations of such monomers. Of thesemonomers, isodecyl acrylate, dodecyl acrylate and 2-ethylhexyl acrylateare the most preferred. The monofunctional monomer(s) will generallycomprise 30 to about 80%, more preferably from about 50 to about 65%, byweight of the monomer component.

The monomer component utilized in the oil phase of the HIPEs alsocomprises one or more monofunctional comonomers capable of impartingtoughness about equivalent to that provided by styrene to the resultingpolymeric foam structure. Tougher foams exhibit the ability to deformsubstantially without failure. These monofunctional comonomer types caninclude styrene-based comonomers (e.g., styrene and ethyl styrene) orother monomer types such as methyl methacrylate where the relatedhomopolymer is well known as exemplifying toughness. The preferredmonofunctional comonomer of this type is a styrene-based monomer withstyrene and ethyl styrene being the most preferred monomers of thiskind. The monofunctional "toughening" comonomer will normally comprisefrom about 5 to about 40%, preferably from about 15% to about 25%, mostpreferably from about 18% about 24%, by weight of the monomer component.

In certain cases, the "toughening" comonomer can also impart the desiredrubber-like properties to the resultant polymer. The C₄ -C₁₂ alkylstyrenes, and in particular p-n-octylstyrene, are examples of suchcomonomers. For such comonomers, the amount of that can be included inthe monomer component will be that of the typical monomer and comonomercombined.

The monomer component also contains a first (and optionally second)polyfunctional crosslinking agent. As with the monofunctional monomersand comonomers, selection of the particular type and amount ofcrosslinking agents is very important to the eventual realization ofpreferred polymeric foams having the desired combination of structural,mechanical, and fluid-handling properties.

The first polyfunctional crosslinking agent can be selected from a widevariety of monomers containing two or more activated vinyl groups, suchas divinylbenzenes, trivinybenzenes, divinyltoluenes, divinylxylenes,divinylnaphthalenes, divinylalkylbenzenes, divinylphenanthrenes,divinylbiphenyls, divinyldiphenylmethanes, divinylbenzyls,divinylphenylethers, divinyldiphenylsulfides, divinylfurans,divinylsulfide, divinyl sulfone, and mixtures thereof. Divinylbenzene istypically available as a mixture with ethyl styrene in proportions ofabout 55:45. These proportions can be modified so as to enrich the oilphase with one or the other component. Generally, it is advantageous toenrich the mixture with the ethyl styrene component while simultaneouslyreducing the amount of styrene in the monomer blend. The preferred ratioof divinylbenzene to ethyl styrene is from about 30:70 and 55:45, mostpreferably from about 35:65 to about 45:55. The inclusion of higherlevels of ethyl styrene imparts the required toughness withoutincreasing the Tg of the resulting copolymer to the degree that styrenedoes. This first cross-linking agent can generally be included in theoil phase of the HIPE in an amount of from about 5% to about 25%, morepreferably from about 12% to about 20%, most preferably from about 12%to about 18%, by weight of the monomer component.

The optional second crosslinking agent can be selected frompolyfunctional acrylates, methacrylates, acrylamides, methacrylamides,and mixtures thereof. These include di-, tri-, and tetra-acrylates, aswell as di-, tri-, and tetramethacrylates, di-, tri-, andtetra-acrylamides, as well as di-, tri-, and tetramethacrylamides; andmixtures of these crosslinking agents. Suitable acrylate andmethacrylate crosslinking agents can be derived from diols, triols andtetraols that include 1,10-decanediol, 1,8-octanediol, 1,6-hexanediol,1,4-butanediol, 1,3-butanediol, 1,4-but-2-enediol, ethylene glycol,diethylene glycol, trimethylolpropane, pentaerythritol, hydroquinone,catechol, resorcinol, triethylene glycol, polyethylene glycol, sorbitol,and the like. (The acrylamide and methacrylamide crosslinking agents canbe derived from the equivalent diamines, triamines and tetramines). Thepreferred diols have at least 2, more preferably at least 4, mostpreferably 6, carbon atoms. This second cross-linking agent cangenerally be included in the oil phase of the HIPE in an amount of from0 to about 15%, preferably from 0 to about 13%, by weight of the monomercomponent.

Without being bound by theory, it is believed this second crosslinkingagent generates a more homogeneously crosslinked structure that developsstrength more efficiently than using either the first or the secondcrosslinker alone at comparable levels. The second crosslinker also hasthe effect of broadening the glass-to-rubber transition region. Thisbroader transition region can be tailored to meet specific strength andresilience requirements at in-use temperatures by controlling therelative amount of the two crosslinker types employed. Thus, a foamcontaining only the first type of crosslinker will exhibit a relativelynarrow transition region. Increasing the amount of the secondcrosslinker serves to broaden the transition region, even if the actualtransition temperature itself has not changed.

The major portion of the oil phase of the HIPEs will comprise theaforementioned monomers, comonomers and crosslinking agents. It isessential that these monomers, comonomers and crosslinking agents besubstantially water-insoluble so that they are primarily soluble in theoil phase and not the water phase. Use of such substantiallywater-insoluble monomers ensures that HIPEs of appropriatecharacteristics and stability will be realized. It is, of course, highlypreferred that the monomers, comonomers and crosslinking agents usedherein be of the type such that the resulting polymeric foam is suitablynon-toxic and appropriately chemically stable. These monomers,comonomers and cross-linking agents should preferably have little or notoxicity if present at very low residual concentrations duringpost-polymerization foam processing and/or use.

Another essential component of the oil phase is an emulsifier componentthat permits the formation of stable HIPEs. This emulsifier componentcomprises a primary emulsifier and optionally a secondary emulsifier.Suitable primary emulsifiers are those which: (1) are soluble in the oilphase of the HIPE; (2) provide a minimum oil phase/water phaseinterfacial tension (IFT) of from about 1 to about 10 dyne/cm,preferably about 2 to about 8 dyne/cm; (3) provide a critical aggregateconcentration (CAC) of about 5 wt. % or less, preferably about 3 wt. %or less; (4) form HIPEs that are sufficiently stable against coalescenceat the relevant drop sizes and the relevant process conditions (e.g.,HIPE formation and polymerization temperatures); and (5) desirably havea high concentration of "interfacially active" component(s) capable oflowering the interfacial tension between the oil and water phases of theHIPE. While not being bound by theory, it is believed that theconcentration of interfacially active components needs to besufficiently high to provide at least approximately monolayer coverageto internal oil phase droplets at the preferred drop sizes, water:oilratios, and emulsifier levels. It is also believed that a combination ofa high minimum oil phase/water phase IFT and low CAC facilitates theformation of a stable HIPE having the suitably-large drop sizes for theformation of a foam having the preferred average cell and hole sizes ofthe present invention. Typically, these primary emulsifiers: (6) havemelt and/or solid-to-liquid crystalline phase-transition temperatures ofabout 30° C. or less; (7) are water dispersible; and (8) aresubstantially water insoluble or at least do not appreciably partitioninto the water phase under the conditions of use. It is preferred thatthe primary emulsifier provide sufficient wettability when spread on ahydrophobic surface (e.g., the polymeric foam) such that the advancingcontact angle for synthetic urine is less than (preferably substantiallyless than) 90°. The method of measurement for IFT and CAC is describedin the TEST METHODS section hereafter.

Especially when used alone, these primary emulsifiers typically compriseat least about 40%, preferably at least about 50%, most preferably atleast about 70%, emulsifying components selected from diglycerolmonoesters of linear unsaturated C₁₆ -C₂₂ fatty acids, diglycerolmonoesters of branched C₁₆ -C₂₄ fatty acids, diglycerol monoaliphaticethers of branched C₁₆ -C₂₄ alcohols, diglycerol monoaliphatic ethers oflinear unsaturated C₁₆ -C₂₂ alcohols, diglycerol monoaliphatic ethers oflinear saturated C₁₂ -C₁₄ alcohols, sorbitan monoesters of linearunsaturated C₁₆ -C₂₂ fatty acids, sorbitan monoesters of branched C₁₆-C₂₄ fatty acids, and mixtures thereof. Preferred primary emulsifiersinclude diglycerol monooleate (e.g., preferably greater than about 40%,preferably greater than about 50%, most preferably greater than about70% diglycerol monooleate) and sorbitan monooleate (e.g., preferablygreater than about 40%, more preferably greater than about 50%, mostpreferably greater than about 70% sorbitan monooleate), and diglycerolmonoisostearate (e.g., preferably greater than about 40%, morepreferably greater than about 50%, most preferably greater than about70% diglycerol monoisostearate).

Diglycerol monoesters of linear unsaturated and branched fatty acidsuseful as emulsifiers in the present invention can be prepared byesterifying diglycerol with fatty acids, using procedures well known inthe art. See, for example, the method for preparing polyglycerol estersdisclosed in U.S. Pat. No. 5,387,207. Diglycerol can be obtainedcommercially or can be separated from polyglycerols that are high indiglycerol. Linear unsaturated and branched fatty acids can be obtainedcommercially. The mixed ester product of the esterification reaction canbe fractionally distilled under vacuum one or more times to yielddistillation fractions that are high in diglycerol monoesters. Forexample, a A CMS-15A (C.V.C. Products Inc.; Rochester, N.Y.) continuous14 inch centrifugal molecular still can be used for fractionaldistillation. Typically, the polyglycerol ester feedstock, while beingheated, is first metered through a degasser unit and then to the heatedevaporator cone of the still, where the vacuum distillation takes place.Distillate is collected on the bell jar surface, which can be heated tofacilitate distillate removal. Distillate and residue are continuouslyremoved by transfer pumps. The fatty acid composition of the resultantmixed ester product can be determined using high resolution gaschromatography. See U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7,1995. Polyglycerol and polyglycerol ester distribution of the resultantmixed ester product can be determined by capillary supercriticalchromatography. See U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7,1995.

Linear saturated, linear unsaturated, or branched diglycerolmonoaliphatic ethers can also be prepared and their compositiondetermined using procedures well known in the art. See also copendingU.S. application Ser. No. 08/514,346 (Stephen A. Goldman et al), filedAug. 9, 1995, which is incorporated by reference.

Sorbitan monoesters of linear unsaturated and branched fatty acids canbe obtained commercially or prepared using methods known in the art.See, for example, U.S. Pat. No. 4,103,047 (Zaki et al), issued Jul. 25,1978 (herein incorporated by reference), especially column 4, line 32 tocolumn 5, line 13. The mixed sorbitan ester product can be fractionallyvacuum distilled to yield compositions that are high in sorbitanmonoesters. Sorbitan ester compositions can be determined by methodswell known in the art such as small molecule gel permeationchromatography. See copending U.S. application Ser. No. 08/514,346(Stephen A. Goldman et al), filed Aug. 9, 1995, which describes the useof this method for polyglycerol aliphatic ethers.

When these primary emulsifiers are used in combination with certainsecondary emulsifiers, the primary emulsifier can comprise lower levelsof these emulsifying components, i.e., as low as about 20% of theseemulsifying components. These secondary emulsifiers are at leastcosoluble with the primary emulsifier in the oil phase and can beincluded to: (1) increase the stability of the HIPE against coalescenceof the dispersed water droplets, especially at higher water-to-oilratios and higher HIPE formation and polymerization temperatures, (2)raise the minimum oil phase/water phase IFT, (3) lower the CAC of theemulsifier component, or (4) increase the concentration of interfaciallyactive components. While not being bound by theory, it is believed thatthe ability of the secondary emulsifier to maintain a high oilphase/water phase IFT and low CAC for the emulsifier component extendsthe range of HIPE formation and pour temperatures (e.g., to about 50° C.or higher) over which a stable high water:oil ratio HIPE can be madethat has the large drop sizes suitable for the formation of polymericfoams having the preferred average cell and hole sizes of the presentinvention. Suitable secondary emulsifiers can be cationic types,including the long chain C₁₂ -C₂₂ dialiphatic, short chain C₁ -C₄dialiphatic quaternary ammonium salts such as ditallow dimethyl ammoniumchloride, bistridecyl dimethyl ammonium chloride, and ditallow dimethylammonium methylsulfate, the long chain C₁₂ -C₂₂dialkoyl(alkenoyl)-2-hydroxyethyl, short chain C₁ -C₄ dialiphaticquaternary ammonium salts such as ditallowoyl-2-hydroxyethyl dimethylammonium chloride, the long chain C₁₂ -C₂₂ dialiphatic imidazoliniumquaternary ammonium salts such as methyl-1-tallow amido ethyl-2-tallowimidazolinium methylsulfate and methyl-1-oleyl amido ethyl-2-oleylimidazolinium methylsulfate, the short chain C₁ -C₄ dialiphatic, thelong chain C₁₂ -C₂₂ monoaliphatic benzyl quaternary ammonium salts suchas dimethyl stearyl benzyl ammonium chloride; anionic types includingthe C₆ -C₁₈ dialiphatic esters of sodium sulfosuccinic acid such as thedioctyl ester of sodium sulfosuccinic acid and the bistridecyl ester ofsodium sulfosuccinic acid; and mixtures of these secondary emulsifiers.These secondary emulsifiers can be obtained commercially or preparedusing methods known in the art. The preferred secondary emulsifiers areditallow dimethyl ammonium methyl sulfate and ditallow dimethyl ammoniummethyl chloride. When these optional secondary emulsifiers are includedin the emulsifier component, it is in a weight ratio of primary tosecondary emulsifier of from about 50:1 to about 1:4, preferably fromabout 30:1 to about 2:1.

The oil phase used to form the HIPEs comprises from about 85 to about98% by weight monomer component and from about 2 to about 15% by weightemulsifier component. Preferably, the oil phase will comprise from about90 to about 97% by weight monomer component and from about 3 to about10% by weight emulsifier component. The oil phase also can contain otheroptional components. One such optional component is an oil solublepolymerization initiator of the general type well known to those skilledin the art, such as described in U.S. Pat. No. 5,290,820 (Bass et al),issued Mar. 1, 1994, which is incorporated by reference. Anotherpreferred optional component is an antioxidant such as a Hindered AmineLight Stabilizer (HALS) and Hindered Phenolic Stabilizers (HPS) or anyother antioxidant compatible with the initiator system to be employed.Other optional components include plasticizers, fillers, colorants,chain transfer agents, dissolved polymers, and the like.

2. Water Phase Components

The discontinuous water internal phase of the HIPE is generally anaqueous solution containing one or more dissolved components. Oneessential dissolved component of the water phase is a water-solubleelectrolyte. The dissolved electrolyte minimizes the tendency of themonomers, comonomers and crosslinkers that are primarily oil soluble toalso dissolve in the water phase. This, in turn, is believed to minimizethe extent to which polymeric material fills the cell windows at theoil/water interfaces formed by the water phase droplets duringpolymerization. Thus, the presence of electrolyte and the resultingionic strength of the water phase is believed to determine whether andto what degree the resulting preferred polymeric foams can beopen-celled.

Any electrolyte capable of imparting ionic strength to the water phasecan be used. Preferred electrolytes are mono-, di-, or trivalentinorganic salts such as the water-soluble halides, e.g., chlorides,nitrates and sulfates of alkali metals and alkaline earth metals.Examples include sodium chloride, calcium chloride, sodium sulfate andmagnesium sulfate. Calcium chloride is the most preferred for use in thepresent invention. Generally the electrolyte will be utilized in thewater phase of the HIPEs in a concentration in the range of from about0.2 to about 20% by weight of the water phase. More preferably, theelectrolyte will comprise from about 1 to about 10% by weight of thewater phase.

The HIPEs will also typically contain a polymerization initiator. Suchan initiator component is generally added to the water phase of theHIPEs and can be any conventional water-soluble free radical initiator.These include peroxygen compounds such as sodium, potassium and ammoniumpersulfates, hydrogen peroxide, sodium peracetate, sodium percarbonateand the like. Conventional redox initiator systems can also be used.Such systems are formed by combining the foregoing peroxygen compoundswith reducing agents such as sodium bisulfite, L-ascorbic acid orferrous salts.

The initiator can be present at up to about 20 mole percent based on thetotal moles of polymerizable monomers present in the oil phase. Morepreferably, the initiator is present in an amount of from about 0.001 toabout 10 mole percent based on the total moles of polymerizable monomersin the oil phase.

3. Hydrophilizing Surfactants and Hydratable Salts

The polymer forming the HIPE foam structure will preferably besubstantially free of polar functional groups. This means the polymericfoam will be relatively hydrophobic in character. These hydrophobicfoams can find utility where the absorption of hydrophobic fluids isdesired. Uses of this sort include those where an oily component ismixed with water and it is desired to separate and isolate the oilycomponent, such as in the case of marine oil spills.

When these foams are to be used as absorbents for aqueous fluids such asjuice spills, milk, and the like for clean up and/or bodily fluids suchas urine, they generally require further treatment to render the foamrelatively more hydrophilic. Hydrophilization of the foam, if necessary,can generally be accomplished by treating the HIPE foam with ahydrophilizing surfactant in a manner described more fully hereafter.

These hydrophilizing surfactants can be any material that enhances thewater wettability of the polymeric foam surface. They are well known inthe art, and can include a variety of surfactants, preferably of thenonionic type. They will generally be liquid form, and can be dissolvedor dispersed in a hydrophilizing solution that is applied to the HIPEfoam surface. In this manner, hydrophilizing surfactants can be adsorbedby the preferred HIPE foams in amounts suitable for rendering thesurfaces thereof substantially hydrophilic, but without substantiallyimpairing the desired flexibility and compression deflectioncharacteristics of the foam. Such surfactants can include all of thosepreviously described for use as the oil phase emulsifier for the HIPE,such as diglycerol monooleate, sorbitan monooleate and diglycerolmonoisostearate. In preferred foams, the hydrophilizing surfactant isincorporated such that residual amounts of the agent that remain in thefoam structure are in the range from about 0.5% to about 15%, preferablyfrom about 0.5 to about 6%, by weight of the foam.

Another material that is typically incorporated into the HIPE foamstructure is a hydratable, and preferably hygroscopic or deliquescent,water soluble inorganic salt. Such salts include, for example,toxicologically acceptable alkaline earth metal salts. Salts of thistype and their use with oil-soluble surfactants as the foamhydrophilizing surfactant is described in greater detail in U.S. Pat.No. 5,352,711 (DesMarais), issued Oct. 4, 1994, the disclosure of whichis incorporated by reference. Preferred salts of this type include thecalcium halides such as calcium chloride that, as previously noted, canalso be employed as the water phase electrolyte in the HIPE.

Hydratable inorganic salts can easily be incorporated by treating thefoams with aqueous solutions of such salts. These salt solutions cangenerally be used to treat the foams after completion of, or as part of,the process of removing the residual water phase from thejust-polymerized foams. Treatment of foams with such solutionspreferably deposits hydratable inorganic salts such as calcium chloridein residual amounts of at least about 0.1% by weight of the foam, andtypically in the range of from about 0.1 to about 12%.

Treatment of these relatively hydrophobic foams with hydrophilizingsurfactants (with or without hydratable salts) will typically be carriedout to the extent necessary to impart suitable hydrophilicity to thefoam. Some foams of the preferred HIPE type, however, are suitablyhydrophilic as prepared, and can have incorporated therein sufficientamounts of hydratable salts, thus requiting no additional treatment withhydrophilizing surfactants or hydratable salts. In particular, suchpreferred HIPE foams include those where certain oil phase emulsifierspreviously described and calcium chloride are used in the HIPE. In thoseinstances, the internal polymerized foam surfaces will be suitablyhydrophilic, and will include residual water-phase liquid containing ordepositing sufficient amounts of calcium chloride, even after thepolymeric foams have been dewatered to a practicable extent.

B. Processing Conditions for Obtaining HIPE Foams

Foam preparation typically involves the steps of: 1) forming a stablehigh internal phase emulsion (HIPE); 2) polymerizing/curing this stableemulsion trader conditions suitable for forming a solid polymeric foamstructure; 3) optionally washing the solid polymeric foam structure toremove the original residual water phase from the polymeric foamstructure and, if necessary, treating the polymeric foam structure witha hydrophilizing surfactant and/or hydratable salt to deposit any neededhydrophilizing surfactant/hydratable salt, and 4) thereafter dewateringthis polymeric foam structure.

1. Formation of HIPE

The HIPE is formed by combining the oil and water phase components inthe previously specified weight ratios. The oil phase will typicallycontain the requisite monomers, comonomers, crosslinkers, andemulsifiers, as well as optional components such as plasticizers,antioxidants, flame retardants, and chain transfer agents. The waterphase will typically contain electrolytes and polymerization initiators,as well as optional components such as water-soluble emulsifiers.

The HIPE can be formed from the combined oil and water phases bysubjecting these combined phases to shear agitation. Shear agitation isgenerally applied to the extent and for a time period necessary to forma stable emulsion. Such a process can be conducted in either batchwiseor continuous fashion and is generally carried out under conditionssuitable for forming an emulsion where the water phase droplets aredispersed to such an extent that the resulting polymeric foam will havethe requisite cell size and other structural characteristics. Suitablemixing or agitation devices are those that are capable of forming anemulsion under conditions of low shear mixing. Emulsification of the oiland water phase combination will frequently involve the use of a mixingor agitation device such as a pin impeller.

One preferred method of forming such HIPEs involves a continuous processthat combines and emulsifies the requisite oil and water phases. In sucha process, a liquid stream comprising the oil phase is formed.Concurrently, a liquid stream comprising the water phase is also formed.The two streams are then combined in a suitable mixing chamber or zonesuch that the requisite water to oil phase weight ratios previouslyspecified are achieved. Such a process is described in copending U.S.application Ser. No. 08/370,694, filed Jan. 10, 1995 by T. DesMarais,which is incorporated by reference herein.

In the mixing chamber or zone (preferably a cylinder), the combinedstreams are generally-subjected to low shear agitation provided, forexample, by a pin impeller of suitable configuration and dimensions.With a pin impeller of the type used in the present process, both theimpeller pin tip speed (hereafter referred to as "tip speed") and thegap between pin tip and the mixing chamber wall (referred to herein as"pin to wall gap", or "gap") are important to shear rate. The shear ratefor pin impellets is herein defined as the tip speed divided by the pinto wall gap. For the purposes of this invention this combinationvariable, shear rate, should be below about 1000 sec⁻¹. Preferably,shear will typically be applied to the combined oil/water phase streamat a rate of about 800 sec⁻¹ or less. Tip speeds should be from about 5in/sec (13 cm/sec) to about 70 in/sec (178 cm/sec), preferably fromabout 5 in/sec (13 cm/sec) to about 60 in/sec (152 cm/sec), morepreferably from about 10 in/sec (25 cm/sec) to about 50 in/sec (127cm/sec). Pin to wall gap should be between 1.5% and 20% of the cylinderdiameter, preferably between 3% and 20% of the cylinder diameter, morepreferably between 5% and 20% of the cylinder diameter.

The low shear described above allows processing at moderate temperatures(e.g., 60° to 70° C.), to obtain foams that have relatively uniform cellsizes. Uniformity of cell size is believed to be important to theability of the foam to desorb. In particular such uniform cell sizes arebelieved to allow the foams to have a desorption at 30 cm of less thanabout 10% of the foam's free absorbent capacity. Once formed, the stableliquid HIPE can then be withdrawn from the mixing chamber or zone. Thispreferred method for forming HIPEs via a continuous process is describedin greater detail in U.S. Pat. No. 5,149,720 (DesMarais et al), issuedSep. 22, 1992, which is incorporated by reference. See also copendingU.S. application Ser. No. 08/370,694 (Thomas A. DesMarais), filed Jan.10, 1995, Case No. 5543 (herein incorporated by reference), whichdescribes an improved continuous process having a recirculation loop forthe HIPE.

One particular advantage of the more robust emulsifier systems used inthese HIPEs is that the mixing conditions during HIPE formation andpouring can be carried out at more elevated temperatures of about 50° C.or higher, preferably 55° C. or higher. Typically, the HIPE can beformed at a temperature of from about 60° to about 99° C., moretypically from about 60° to about 95° C.

2. Polymerization/Curing of the HIPE

The HIPE formed will generally be collected or poured into a suitablereaction vessel, container or region to be polymerized or cured. In oneembodiment the reaction vessel comprises a tub constructed ofpolyethylene from which the eventually polymerized/cured solid foammaterial can be easily removed for further processing afterpolymerization/curing has been carried out to the extent desired. It isusually preferred that the temperature at which the HIPE is poured intothe vessel be approximately the same as the polymerization/curingtemperature.

Suitable polymerization/curing conditions will vary depending upon themonomer and other makeup of the oil and water phases of the emulsion(especially the emulsifier systems used), and the type and amounts ofpolymerization initiators used. Frequently, however, suitablepolymerization/curing conditions will involve maintaining the HIPE atelevated temperatures above about 50° C., more preferably above about65° C., and most preferably above about 80° C., for a time periodranging from about 2 to about 64 hours, more preferably from about 2 toabout 48 hours. An advantage of the more robust emulsifier systems usedis that coalescence is minimized when polymerization/curing is carriedout at higher temperatures. The HIPE can also be cured in stages such asdescribed in U.S. Pat. No. 5,189,070 (Brownscombe et al), issued Feb.23, 1993, which is herein incorporated by reference.

A porous water-filled open-celled HIPE foam is typically obtained afterpolymerization/curing in a reaction vessel, such as a tub. Thispolymerized HIPE foam is typically cut or sliced into a sheet-like form.Sheets of polymerized HIPE foam are easier to process during subsequenttreating/washing and dewatering steps, as well as to prepare the HIPEfoam for use in absorbent articles. The polymerized HIPE foam istypically cut/sliced to provide a cut thickness in the range of fromabout 0.08 to about 2.5 cm.

3. Treating/Washing HIPE Foam

The solid polymerized HIPE foam formed will generally be filled withresidual water phase material used to prepare the HIPE. This residualwater phase material (generally an aqueous solution of electrolyte,residual emulsifier, and polymerization initiator) should be at leastpartially removed prior to further processing and use of the foam.Removal of this original water phase material will usually be carriedout by compressing the foam structure to squeeze out residual liquidand/or by washing the foam structure with water or other aqueous washingsolutions. Frequently several compressing and washing steps, e.g., from2 to 4 cycles, will be used.

After the original water phase material has been removed to the extentrequired, the HIPE foam, if needed, can be treated, e.g., by continuedwashing, with an aqueous solution of a suitable hydrophilizingsurfactant and/or hydratable salt. Hydrophilizing surfactants andhydratable salts that can be employed have been previously described. Asnoted, treatment of the HIPE foam with the hydrophilizingsurfactant/hydratable salt solution continues, if necessary, until thedesired amount of hydrophilizing surfactant/hydratable salt has beenincorporated and until the foam exhibits the desired adhesion tensionvalue for any test liquid of choice.

For certain absorbent uses, removal of most of the residual electrolyte(i.e., hydratable salts) from the foam can be desirable. For example,removal of these salts is particularly important when the foam is to beused in an absorbent core (as described hereafter) that also has a fluidstorage component that contains absorbent gelling materials. In thesecircumstances, the level of these residual hydratable salts in the foamis reduced as much as possible during this washing step, typically toabout 2% or less, preferably to about 0.5% or less. After the removal ofthese salts, the HIPE foam will typically require treatment with aneffective amount of a suitable hydrophilizing surfactant torehydrophilize the foam.

4. Foam Dewatering

After the HIPE foam has been treated/washed, it will generally bedewatered. Dewatering can be achieved by compressing the foam(preferably in the z-direction) to squeeze out residual water, bysubjecting the foam and the water therein to temperatures of from about60° to about 200° C., or to microwave treatment, by vacuum dewatering orby a combination of compression and thermal drying/microwave/vacuumdewatering techniques. The dewatering step will generally be carried outuntil the HIPE foam is ready for use and is as dry as practicable.Frequently such compression dewatered foams will have a water (moisture)content of from about 50 to about 500%, more preferably from about 50 toabout 200%, by weight on a dry weight basis. Subsequently, thecompressed foams can be thermally dried to a moisture content of fromabout 5 to about 40%, more preferably from about 5 to about 15%, on adry weight basis.

III. Uses of Polymeric Foams

A. In General

Polymeric foams according to the present invention are broadly useful inabsorbent cores of disposable diapers, as well as other absorbentarticles. These foams can also be employed as environmental waste oilsorbents; as absorbent components in bandages or dressings; to applypaint to various surfaces; in dust mop heads; in wet mop heads; indispensers of fluids; in packaging; in shoes as odor/moisture sorbents;in cushions; in gloves, and for many other uses.

B. Absorbend Articles

Absorbent foams of the present invention are particularly useful as atleast a portion of the absorbent structures (e.g., absorbent cores) forvarious absorbent articles. By "absorbent article" herein is meant aconsumer product that is capable of absorbing significant quantities ofurine, or other fluids like aqueous fecal matter (runny bowelmovements), discharged by an incontinent wearer or user of the article.Examples of such absorbent articles include disposable diapers,incontinence garments, catamenials such as tampons and sanitary napkins,disposable training pants, bed pads, and the like. The absorbent foamstructures herein are particularly suitable for use in articles such asdiapers, sanitary napkins, tampons, incontinence pads or garments,clothing shields, and the like.

The absorbent foams of the present invention provide good aesthetics dueto their soft, resilient structure and physical integrity. In sheetform, these absorbent foams can also be relatively easy to configure foruse in a variety of absorbent articles. In contrast to fibrous absorbentcomponents, these absorbent foams remain largely unchanged in overallappearance and structure during use, i.e. density, shape, thickness,etc. Since these absorbent foams are not plasticized by aqueous fluids,their mechanical properties remain largely unchanged when wet.

Because the foams of the present invention rapidly acquire anddistribute aqueous fluids, they are particularly useful as the fluidacquisition/distribution component of an absorbent core. Theseacquisition/distribution foams combine relatively high capillaryabsorption pressures and capacity-per-weight properties that allows themto acquire fluid with or without the aid of gravity, therefore keepingthe wearer's skin dry. This high capacity (per given weight) can lead tolight-weight, efficient products.

In addition, because the absorbent foams of the present invention cangive up this acquired fluid efficiently to other absorbent components,these foams are particularly useful as the upperacquisition/distribution component in a "multi-layer" absorbent corethat additionally contains a lower fluid storage/redistributioncomponent, where the absorbent core is positioned between the topsheetand backsheet to form the absorbent article. For purposes of the presentinvention, an "upper" layer of a multi-layer absorbent core is a layerthat is relatively closer to the body of the wearer, e.g., the layerclosest to the article topsheet. The term "lower" layer conversely meansa layer of a multi-layer absorbent core that is relatively further awayfrom the body of the wearer, e.g., the layer closest to the articlebacksheet. This lower fluid storage/redistribution layer is typicallypositioned within the absorbent core so as to underlie the (upper) fluidacquisition/distribution layer and be in fluid communication therewith.This lower storage/redistribution layer can comprise a variety of fluidstorage/redistribution components including those containing absorbentgelling materials such as disclosed in U.S. Pat. No. 5,061,259 (Goldmanet. al), issued Oct. 29, 1991, U.S. Pat. No. 4,654,039 (Brandt et al),issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Pat. No. Re.32,649), U.S. Pat. No. 4,666,983 (Tsubakimoto et al), issued May 19,198, and U.S. Pat. No. 4,625,001 (Tsubakimoto et al), issued Nov. 25,1986, all of which are incorporated by reference; absorbentmacrostructures made from these absorbent gelling materials such asthose disclosed in U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7,1992, and U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994,both of which are incorporated by reference); absorbent gellingmaterials laminated between two tissue layers such as those disclosed inU.S. Pat. No. 4,260,443 (Lindsay et al), issued Apr. 7, 1981, U.S. Pat.No. 4,467,012 (Pedersen et al), issued Aug. 21, 1984, U.S. Pat. No.4,715,918 (Lang), issued Dec. 29, 1987, U.S. Pat. No. 4,851,069 (Packardet al), issued Jul. 25, 1989, U.S. Pat. No. 4,950,264 (Osborn), issuedAug. 21, 1990; U.S. Pat. No. 4,994,037 (Bernardin), issued Feb. 19,1991; U.S. Pat. No. 5,009,650 (Bernardin), issued Apr. 23, 1991; U.S.Pat. No. 5,009,653 (Osborn), issued Apr. 23, 1991; U.S. Pat. No.5,128,082 (Makoui), Jul. 7, 1992; U.S. Pat. No. 5,149,335 (Kellenbergeret al), issued Sep. 22, 1992; and U.S. Pat. No. 5,176,668(Bernardin),issued Jan. 5, 1993, all of which are incorporated byreference; and absorbent foams capable of storing acquired fluids suchas those disclosed in U.S. Pat. No. 5,268,224 (DesMarais et al.), issuedDec. 7, 1993; U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7, 1995;and copending U.S. application Ser. No. 08/370,922 (Thomas A. DesMaraiset al), filed Jan. 10, 1995, Case No. 5541; all of which areincorporated by reference.

There is no particular criticality with respect to the positionalrelationship of the fluid acquisition/distribution foam component andthe fluid storage/redistribution component within these multi-layerabsorbent cores so long as these components are in effective fluidcommunication with each other and so long as each component is largeenough to effectively hold and/or transport the amount of aqueous bodyfluid that is expected to be discharged into the absorbent article. Onesuitable relationship between the fluid acquisition/distribution foamcomponent and the fluid storage/redistribution component within theabsorbent core is to place these components in a layered configuration.In such a layered configuration, the fluid acquisition/distribution foamcomponent comprises an upper foam layer which overlies a subjacent fluidstorage/redistribution component in the form of a lower layer. It shouldbe understood that these two types of layers refer merely to the upperand lower zones of the absorbent core and are not necessarily limited tosingle layers or sheets. Both the fluid acquisition/distribution zone,e.g., upper layer, and the fluid storage/redistribution zone, e.g.,lower layer, can comprise several layers of the requisite type. Thus, asused herein, the term "layer" includes the terms "layers" and "layered."

The absorbent articles typically comprise a fluid impervious backsheet,a fluid pervious topsheet joined to, or otherwise associated with thebacksheet, and an absorbent core according to the present inventionpositioned between the backsheet and the topsheet. The topsheet ispositioned adjacent the body surface of the absorbent core. The topsheetis preferably joined to the backsheet by attachment means such as thosewell known in the art. As used herein, the term "joined" encompassesconfigurations whereby an element is directly secured to the otherelement by affixing the element directly to the other element, andconfigurations whereby the element is indirectly secured to the otherelement by affixing the element to intermediate member(s) which in mmare affixed to the other element. In preferred absorbent articles, thetopsheet and the backsheet are joined directly to each other at theperiphery thereof.

The backsheet is typically impervious to body fluids and is preferablymanufactured from a thin plastic film, although other flexible fluidimpervious materials may also be used. As used herein, the term"flexible" refers to materials that are compliant and will readilyconform to the general shape and contours of the human body. Thebacksheet prevents body fluids absorbed and contained in the absorbentcore from wetting clothes that contact the articles such as pants,pajamas, undergarments, and the like. The backsheet can comprise a wovenor nonwoven material, polymeric films such as thermoplastic films ofpolyethylene or polypropylene, or composite materials such as afilm-coated nonwoven material. Preferably, the backsheet is apolyethylene film having a thickness of from about 0.012 mm (0.5 mil) toabout 0.051 mm (2.0 mils). Exemplary polyethylene films are manufacturedby Clopay Corporation of Cincinnati, Ohio, under the designationP18-0401 and by Ethyl Corporation, Visqueen Division, of Terre Haute,Ind., under the designation XP-39385. The backsheet is preferablyembossed and/or matte finished to provide a more clothlike appearance.Further, the backsheet can permit vapors to escape from the absorbentcore (i.e., breathable) while still preventing body fluids from passingthrough the backsheet.

The topsheet is compliant, soft feeling, and non-irritating to thewearer's skin. Further, the topsheet is fluid pervious permitting bodyfluids to readily penetrate through its thickness. A suitable topsheetcan be manufactured from a wide range of materials such as woven andnonwoven materials; polymeric materials such as apertured formedthermoplastic films, apertured plastic films, and hydroformedthermoplastic-films; porous foams; reticulated foams; reticulatedthermoplastic films; and thermoplastic scrims. Suitable woven andnonwoven materials can be comprised of natural fibers (e.g., wood orcotton fibers), synthetic fibers (e.g., polymeric fibers such aspolyester, polypropylene, or polyethylene fibers) or from a combinationof natural and synthetic fibers.

Preferred topsheets for use in absorbent articles of the presentinvention are selected from high loft nonwoven topsheets and aperturedformed film topsheets. Apertured formed films are especially preferredfor the topsheet because they are pervious to body fluids and yetnon-absorbent and have a reduced tendency to allow fluids to pass backthrough and rewet the wearer's skin. Thus, the surface of the formedfilm that is in contact with the body remains dry, thereby reducing bodysoiling and creating a more comfortable feel for the wearer. Suitableformed films are described in U.S. Pat. No. 3,929,135 (Thompson), issuedDec. 30, 1975; U.S. Pat. No. 4,324,246 (Mullane, et al.), issued Apr.13, 1982; U.S. Pat. No. 4,342,314 (Radel. et al.), issued Aug. 3, 1982;U.S. Pat. No. 4,463,045 (Ahr et al.), issued Jul. 31, 1984; and U.S.Pat. No. 5,006,394 (Baird), issued Apr. 9, 1991. Each of these patentsare incorporated herein by reference. Suitable microapertured formedfilm topsheets are disclosed in U.S. Pat. No. 4,609,518 (Curro et al),issue Sep. 2, 1986 and U.S. Pat. No. 4,629,643 (Curro et al), issuedDec. 16, 1986, which are incorporated by reference.

The body surface of the formed film topsheet can be hydrophilic so as tohelp body fluids to transfer through the topsheet faster than if thebody surface was not hydrophilic so as to diminish the likelihood thatfluid will flow off the topsheet rather than flowing into and beingabsorbed by the absorbent structure. In a preferred embodiment,surfactant is incorporated into the polymeric materials of the formedfilm topsheet such as is described in U.S. patent application Ser. No.07/794,745, "Absorbent Article Having A Nonwoven and Apertured FilmCoversheet" filed on Nov. 19, 1991 by Aziz, et al., which isincorporated by reference. Alternatively, the body surface of thetopsheet can be made hydrophilic by treating it with a surfactant suchas is described in the above referenced U.S. Pat. No. 4,950,254,incorporated herein by reference.

In some embodiments according to the present invention, theacquisition/distribution layer of the absorbent core will be placed in aspecific positional relationship with respect to the topsheet and thestorage/redistribution layer of the absorbent core. More particularly,the acquisition/distribution layer of the core is positioned so that itis effectively located to acquire discharged body fluid and transportsuch fluid to other regions of the core. Thus theacquisition/distribution layer can encompass the vicinity of the pointof discharge of body fluids. These areas would include the crotch areaand, preferably for articles to be worn by males, also the region whereurination discharges occur in the front of the diaper. For a diaper, thefront of the absorbent articles means the portion of the absorbentarticle which is intended to be placed on the front of the wearer.Additionally, for male wearers, it is desirable for theacquisition/distribution layer to extend to near the front waist area ofthe wearer to effectively acquire the relatively high fluid load thatoccurs in the front of diapers for male wearers, and to compensate fordirectional variations of the discharges. The corresponding absorbentarticle regions can vary depending upon the design and fit of theabsorbent article.

For diaper executions, the acquisition/distribution layer of the corecan be positioned relative to an elongated topsheet and/or thestorage/redistribution layer such that the acquisition/distributionlayer is of sufficient length to extend to areas corresponding at leastto about 50%, preferably 75%, of the length of the topsheet and/or fromabout 50 to about 120% of the length of the storage/redistributionlayer. The acquisition/distribution foam layer should have a widthsufficient to acquire gushes of body fluids and to prevent directdischarge of fluid onto the storage/redistribution layer. Generally, fordiapers, the width of the acquisition/distribution layer will be atleast about 5 cm, preferably at least about 6 cm.

For purposes of determining such acquisition/distribution foam layerpositioning, the length of the absorbent article will be taken as thenormal longest longitudinal dimension of the elongated article backingsheet. This normal longest dimension of the elongated backing sheet canbe defined with respect to the article as it is applied to the wearer.When worn, the opposing ends of the back sheet are fastened together sothat these joined ends form a circle around the wearer's waist. Thenormal length of the backing sheet will thus be the length of the linerunning through the back sheet from a) the point on the edge of the backsheet at the middle of the wearer's back waist, through the crotch, tob) the point on the opposite edge of the backing sheet at the middle ofthe wearer's front waist. The size and shape of the topsheet willgenerally correspond substantially to the back sheet.

In the usual instance, the storage/redistribution layer of the absorbentcores which generally defines the shape of the absorbent article and thenormal length of the elongated article topsheet will be approached bythe longest longitudinal dimension of the storage/redistribution layerof the core. However, in some articles (e.g., adult incontinencearticles) where bulk reduction or minimum cost are important, thestorage/redistribution layer would be generally located to cover onlythe genital region of the wearer and a reasonable area proximate to thegenital area. In this instance both the fluid acquisition/distributionlayer and the storage/redistribution layer would be located toward thefront of the article as defined by the topsheet such that theacquisition/distribution and storage/redistribution layers wouldtypically be found in the front two-thirds of the article length.

The acquisition/distribution foam layer can be of any desired shapeconsistent with comfortable fit and the sizing limitations discussedabove. These shapes include, for example, circular, rectangular,trapezoidal or oblong, e.g., hourglass-shaped, dog-bone-shaped, half dogbone shaped, oval or irregularly shaped. The acquisition/distributionfoam layer can be of similar shape or differing shape than thestorage/redistribution layer. The storage/redistribution layer of thepreferred absorbent core configuration can also be of any desired shapeconsistent with comfortable fit including, for example, circular,rectangular, trapezoidal or oblong, e.g., hourglass-shaped,dog-bone-shaped, half dog bone shaped, oval or irregularly shaped. Thestorage/redistribution layer need not be physically separated from theacquisition/distribution layer or completely unattached from thestorage/redistribution layer.

FIGS. 5 and 6 show a multi-layer absorbent core configuration where thefluid storage/redistribution component comprises a generallyrectangularly-shaped top layer 64 which is placed over an underlyinghourglass-shaped fluid acquisition/distribution lower foam layer 65. Thefluid storage/redistribution layer contains a fluid acquisition aperture66 through which body fluid is discharged so as to impinge on thesubjacent acquisition/distribution lower layer 65.

FIG. 7 shows a disposable diaper having another multi-layer absorbentcore configuration. Such a diaper comprises a topsheet, 70, afluid-impervious backsheet, 71, and a dual layer absorbent corepositioned between the topsheet and the backsheet. The dual layerabsorbent core comprises a modified hourglass-shaped, fluidstorage/redistribution layer 72 positioned below a modified-hourglassshaped fluid acquisition/distribution foam layer, 73. The topsheetcontains two substantially parallel barrier leg cuff strips 74 withelastic. Affixed to the diaper backsheet are two rectangular elasticizedwaistband member 75. Also affixed to each end of the backsheet are twowaistshield elements 76. Also affixed to backsheet are two parallel legelastic strips 77. A sheet 78 is affixed to the outside of the backsheetas a dedicated fastening surface for two pieces 79 of Y-tape which canbe used to fasten the diaper around the wearer.

Multi-layer absorbent cores can also be made according to copending U.S.application Ser. No. 08/521,556 (Gary D. LaVon et al), filed Aug. 30,1995, (herein incorporated by reference), where one or more layerscomprise an absorbent foam according to the present invention.

IV. Test Methods

A. Capillary Absorption Pressure

A capillary absorption isotherm curve is generated using the VerticalWicking Absorbent Capacity test described in the TEST METHODS section ofU.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7, 1995, which isincorporated by reference, except at 31° C. rather than 37° C. The curveis a plot of the absorbent capacity of each segment as a function ofwicked height, using the distance from the top of the water reservoir tothe midpoint of each segment for the height h. The capillary absorptionpressure is taken as the height of the foam that has an absorbentcapacity one-half of the foam's free absorbent capacity.

B. Capillary Desorption Pressure

Capillary desorption pressure is a measure of the foam's ability to holdonto fluid as a function of various hydrostatic heads. The sample stripof suitable dimensions, e.g., 40 cm long×2.5 cm wide×0.2 cm thick, andthe test liquid (distilled water, optionally containing a small amountof food coloring as indicator), are equilibrated in a room at 22°±2° C.The measurement is carried out at this same temperature.

The foam strip is saturated in water, then positioned vertically suchthat the lower end is immersed 1-2 mm in a reservoir of water. The wateris allowed to drain from the sample until equilibrium is reached,typically 16-24 hours. During this procedure, the sample and reservoirshould be shielded, for example by using a glass cylinder and aluminumfoil, to prevent water loss due to evaporation. The sample is thenquickly removed and placed on a non-absorbent surface where it is cutinto 2.5 cm segments after discarding the portion of the sample that wasimmersed in the reservoir. Each piece is weighed, washed with water,dried and then reweighed. The absorbent capacity is calculated for eachpiece.

A capillary desorption isotherm curve is generated by plotting theabsorbent capacity of each segment as a function of height. The curve isa plot of the absorbent capacity of each segment as a function of heightthat the test fluid desorbed, using the distance from the top of thewater reservoir to the midpoint of each segment for the height h. Thecapillary desorption pressure is taken as the height of the foam thathas an absorbent capacity one-half of the foam's free absorbentcapacity.

C. Resistance to Compression Deflection (RTCD)

Resistance to compression deflection can be quantified by measuring theamount of strain (% reduction in thickness) produced in a foam samplewhich has been saturated with synthetic urine, after a confiningpressure of 0.74 psi (5.1 kPa) has been applied to the sample.Resistance to Compression Deflection measurements are typically made onthe same sample concurrently with the measurement of Free AbsorbentCapacity as described below.

Jayco synthetic urine used in this method is prepared by dissolving amixture of 2.0 g KCl, 2.0 g Na₂ SO₄, 0.85 g NH₄ H₂ PO₄, 0.15 g (NH₄)₂HPO₄, 0.19 g CaCl₂, and 0.23 g MgCl₂ to 1.0 liters with distilled water.The salt mixture can be purchased from Endovations, Reading, Pa. (catNo. JA-00131-000-01).

The foam samples, Jayco synthetic urine and equipment used to makemeasurements are all equilibrated to a temperature of 31° C. Allmeasurements are also performed at this temperature.

A foam sample sheet is saturated to its free absorbent capacity bysoaking in a bath of Jayco synthetic urine. After 3 minutes, a cylinderhaving a 1 in² (6.5 cm²) circular surface area is cut out of thesaturated sheet with a sharp circular die. The cylindrical sample issoaked in synthetic urine at 31° C. for a further 6 minutes. The sampleis then removed from the synthetic urine and is placed on a flat granitebase under a gauge suitable for measuring the sample thickness. Thegauge is set to exert a pressure of 0.08 psi (0.55 kPa) on the sample.Any gauge fitted with a foot having a circular surface area of at least1 in² (6.5 cm²) and capable of measuring thickness to 0.001 in (0.025mm) can be employed. Examples of such gauges are an Ames model 482 (AmesCo.; Waltham, Mass.) or an Ono-Sokki model EG-225 (Ono-Sokki Co., Ltd.;Japan).

After 2 to 3 min., the expanded thickness (X1) is recorded. A force isthen applied to the foot so that the saturated foam sample is subjectedto a pressure of 0.74 psi (5.1 kPa) for 15 minutes. At the end of thistime, the gauge is used to is measure the final sample thickness (X2).From the initial and final thickness measurements, the percent straininduced can be calculated for the sample as follows: (X1-X2)/X1!×100=%reduction in thickness.

D. Recovery from Wet Compression (RFWC)

The foam samples, Jayco synthetic urine and equipment used to makemeasuremental are all equilibrated at 31° C. and 50% relative humidity.All measurements are also performed at this temperature and humidity.Thickness measurements are performed under a pressure of about 0.08 psi(0.55 kPa) using a gauge such as an Ames model 482 or an Ono-Sokki modelEG-225.

A foam cylinder about 2 mm thick and 29 mm diameter is punched out of asheet of foam. It is saturated to its free absorbent capacity in Jaycosynthetic urine, then placed on top of three sheets of 9 cm diameterWhatman Grade No. 3 filter paper (particle retention: 6 μm). The role ofthe filter paper is to simulate the high absorption pressures typicallyassociated with storage components in absorbent articles.

The foam/paper composite is immediately compressed 75% of the thicknessof the wet foam (1.5 mm for a 2 mm thick sample) using a rigid platelarger in area than the foam sample. This strain is maintained for fiveminutes, during which time most of the synthetic urine is partitionedout of the foam and into the filter paper. After the five minute period,the confining plate is removed from the foam/paper composite, and thefoam is given the opportunity to imbibe air and reexpand. Two minutesafter removing the confining plate, the sample is separated from thepaper and its thickness measured. The extent to which the samplerecovers its thickness, expressed as a percentage of its initialthickness, is taken as a measure of the recovery from wet compression ofthe sample. The average of at least three measurements are used todetermine RFWC.

E. Free Absorbent Capacity (FAC)

Free absorbent capacity can be quantified by measuring the amountsynthetic urine absorbed in a foam sample which has been saturated withsynthetic urine. Free Absorbent Capacity measurements are typically madeon the same sample concurrently with the measurement of Resistance toCompression Deflection.

The foam samples and Jayco synthetic urine are equilibrated to atemperature of 31° C. Measurements are performed at ambient temperature.

A foam sample sheet is saturated to its free absorbent capacity bysoaking in a bath of Jayco synthetic urine. After 3 minutes, a cylinderhaving a 1 in² (6.5 cm²) circular surface area is cut out of thesaturated, expanded sheet with a sharp circular die. The cylindricalsample is soaked in synthetic urine at 31° C. for a further 3 minutes.The sample is then removed from the synthetic urine and is placed on adigital balance. Any balance fitted with a weighing pan having an arealarger than that of the sample and with a resolution of 1 milligram orless can be employed. Examples of such balances are the Mettier PM 480and Mettler PC 440 (Mettier Instrument Corp; Hightstown, N.J.).

After determining the weight of the wet foam sample (Ww), it is placedbetween 2 fine plastic mesh screens on top of 4 disposable paper towels.The sample is squeezed 3 times by firmly rolling a plastic roller overthe top screen. The sample is then removed, soaked in distilled waterfor approximately 2 minutes, and squeezed between mesh screens asbefore. It is then placed between 8 layers of disposable paper towels (4on each side) and pressed with 20,000 lbs. of force in a CarverLaboratory Press. The sample is then removed from the paper towels,dried in an oven at 82° C. for 20 minutes, and its dry weight recorded(Wd).

The free absorbent capacity (FAG) is the wet weight (Ww), less the dryweight (Wd) divided by the dry weight (Wd), i.e., FAG= (Ww-Wd)/Wd!.

F. Dynamic Mechanical Analysis (DMA)

DMA is used to determine the Tgs of polymers including polymeric foams.Samples of the foams are sliced into blocks 3-5 mm in thickness andwashed 3-4 times in distilled water, expressing the fluid through rollernips between each washing. The resulting foam blocks are allowed to dryin air. The dried foam slices are cored to yield a cylinders 25 mm indiameter. These cylinders are analyzed using a Rheometrics RSA-IIdynamic mechanical analyzer set in compression mode using parallelplates 25 mm in diameter. Instrument parameters used were as follows:

Temperature step from ca. 85° C. to -40° C. in steps of 2.5° C.

Soak intervals between temperature changes of 125-160 seconds

Dynamic strain set at 0.1% to 1.0% (usually 0.7%)

Frequency set at 1.0 radians/second

Autotension set in static force tracking dynamic force mode with initialstatic force set at 5 g.

The glass transition temperature is taken as the maximum point of theloss tangent versus temperature curve.

G. Interfacial Tension (IFT) Method (Spinning Drop)

Interfacial Tension (IFT) is measured at 50° C. by the spinning dropmethod described in U.S. Pat. No. 5,387,207 (Dyer et al), issued Feb. 7,1995, except that: (1) the monomer mixture used in preparing the oilphase contains styrene, divinylbenzene (55% technical grade),2-ethylhexylacrylate, and 1,4-butanediol dimethacrylate in a weightratio of 14:14:60:12; (2) the concentration of emulsifier in the oilphase is varied by dilution from an upper concentration of generallyabout 5-10 weight % down to a concentration where the IFT increases to avalue that is at least about 10 dyne/cm greater than the minimum IFT, orabout 18 dyne/cm, whichever is less; (3) a smooth line drawn through aplot of IFT versus log emulsifier concentration is used to determine theminimum IFT; (4) the Critical Aggregation Concentration (CAC) isdetermined by extrapolating the low-concentration, generally linearportion of the IFT versus log concentration plot (i.e., the portion ofthe curve typically used to calculate surface area per molecule at theinterface, see for example Surfactants and Interfacial Phenomena, SecondEdition, Milton J. Rosen, 1989, Pages 64-69) to higher concentration;the emulsifier concentration on this extrapolated line corresponding tothe minimum IFT is taken as the CAC. Generally, an upper emulsifierconcentration of about 5-10 weight % is used. Desirably, the upperemulsifier concentration used is at least about twice (more desirably atleast about three times) the CAC of the emulsifier. For emulsifiershaving a solubility in the oil phase at ambient-temperature of less than5 wt. %, the upper concentration limit can be reduced as long as thisconcentration is still at least about twice the CAC of the emulsifier at50° C.

V. SPECIFIC EXAMPLES

These examples illustrate the specific preparation of collapsed HIPEfoams of the present invention.

Example 1 Preparation of HIPE and Foams from a HIPE

A) HIPE Preparation

Anhydrous calcium chloride (36.32 kg) and potassium persulfate (189 g)are dissolved in 378 liters of water. This provides the water phasestream to be used in a continuous process for forming a HIPE emulsion.

To a monomer combination comprising low purity divinylbenzene (39%divinylbenzene and 61% ethyl styrene) (1912.5 g), commercial divinylbenzene (55% divinylbenzene) (607.5), and 2-ethylhexyl acrylate (3480g), is added a diglycerol monooleate emulsifier (360 g), and Tinuvin 765(30 g). The diglycerol monooleate emulsifier (Grindsted Products;Brabrand, Denmark) comprises approximately 81% diglycerol monooleate, 1%other diglycerol monoesters, 3% polyols, and 15% other polyglycerolesters, imparts a minimum oil/water interfacial tension value ofapproximately 2.7 dyne/cm and has an oil/water critical aggregationconcentration of approximately 2.8 wt %. After mixing, this combinationof materials is allowed to settle overnight. No visible residue isformed and all of the mixture is withdrawn and used as the oil phase ina continuous process for forming a HIPE emulsion.

Separate streams of the oil phase (25° C.) and water phase (68°-70° C.)are fed to a dynamic mixing apparatus. Thorough mixing of the combinedstreams in the dynamic mixing apparatus is achieved by means of a pinimpeller. The pin impeller comprises a cylindrical shaft of about 21.6cm in length with a diameter of about 1.9 cm. The shaft holds 4 rows ofpins, 2 rows having 17 pins and 2 rows having 16 pins, each having adiameter of 0.5 cm extending outwardly from the central axis of theshaft to a length of 1.5 cm. The pin impeller is mounted in acylindrical sleeve which forms the dynamic mixing apparatus, and thepins have a clearance of 1.5 mm from the walls of the cylindricalsleeve.

A minor portion of the effluent exiting the dynamic mixing apparatus iswithdrawn and enters a recirculation zone, as shown in FIG. 1 ofcopending U.S. application Ser. No. 08/370,694, filed Jan. 10, 1995 (T.DesMarais), which is incorporated by reference herein. The Waukesha pumpin the recirculation zone returns the minor portion to the entry pointof the oil and water phase flow streams to the dynamic mixing zone.

A spiral static mixer is mounted downstream from the dynamic mixingapparatus to provide back pressure in the dynamic mixing apparatus andto provide improved incorporation of components into the HIPE that iseventually formed. The static mixer (TAH Industries Model 100-612 is14.7 inches (37.3 cm) long with a 1.05 inch (2.67 cm) outside diameter.

The combined mixing and recirculation apparatus set-up is filled withoil phase and water phase at a ratio of 4 parts water to 1 part oil. Thedynamic mixing apparatus is vented to allow air to escape while fillingthe apparatus completely. The flow rates during filling are 1.89 g/secoil phase and 7.57 cc/sec water phase. When the apparatus is filledrecirculation is begun at about 9.5 cc/sec recirculation.

Once the apparatus set-up is filled, the water phase flow rate is cut to5.67 cc/sec to reduce the pressure build up while the vent is closed.Agitation is then begun in the dynamic mixer, with the impeller miningat 500 RPM. The flow rate of the water phase is then steadily increasedto a rate of 37.8 cc/sec over a time period of about 1 min., and the oilphase flow rate is reduced to 0.95 g/see over a time period of about 3min. The recirculation rate is steadily increased to about 39 cc/secduring the latter time period. The back pressure created by the dynamicand static mixers at this point is about 2 PSI (14 kPa). The Waukeshapump speed is then steadily decreased to a yield a recirculation rate ofabout 20 cc/sec, and the impeller speed is steadily decreased to 300RPM. The back pressure at this point is about 1.7 PSI (12 kPa).

B) Polymerization of HIPE

The HIPE flowing from the static mixer at this point is collected in around polypropylene tub, 17 in. (43 cm) in diameter and 7.5 in (10 cm)high, with a concentric insert made of Celcon plastic. The insert is 5in (12.7 cm) in diameter at its base and 4.75 in (12 cm) in diameter atits top and is 6.75 in (17.1 cm) high. The HIPE-containing tubs are keptin a room maintained at 65° C. for 18 hours to bring aboutpolymerization and form the foam.

C) Foam Washing and Dewatering

The cured HIPE foam is removed from the curing tubs. The foam at thispoint has residual water phase (containing dissolved emulsifiers,electrolyte, initiator residues, and initiator) about 40-50 times(40-50×) the weight of polymerized monomers. The foam is sliced with asharp reciprocating saw blade into sheets which are 0.075 inches (0.19cm) in thickness. These sheets are then subjected to compression in aseries of 2 porous nip rolls equipped with vacuum which gradually reducethe residual water phase content of the foam to about 1 times (1×) theweight of the polymerized material. At this point, the sheets are thenresaturated with a 1.5% CaCl₂ solution at 60° C., are squeezed in aseries of 3 porous nip rolls equipped with vacuum to a water phasecontent of about 1×. The CaCl₂ content of the foam is between 1 and 3%.

Example 2 Preparation of HIPE and Foams from a HIPE

A) HIPE Preparation

Anhydrous calcium chloride (36.32 kg) and potassium persulfate (189 g)are dissolved in 378 liters of water. This provides the water phasestream to be used in a continuous process for forming a HIPE emulsion.

To a monomer combination comprising low purity divinylbenzene (39%divinylbenzene and 61% ethyl styrene) (1912.5 g), commercial divinylbenzene (55% divinylbenzene) (607.5 g), and 2-ethylhexyl acrylate (3480g), is added a diglycerol monooleate emulsifier (360 g), and Tinuvin 765(30 g). The diglycerol monooleate emulsifier (Grindsted Products;Brabrand, Denmark) comprises approximately 81% diglycerol monooleate, 1%other diglycerol monoesters, 3% polyols, and 15% other polyglycerolesters, imparts a minimum oil/water interfacial tension value ofapproximately 2.7 dyne/cm and has an oil/water critical aggregationconcentration of approximately 2.8 wt %. After mixing, this combinationof materials is allowed to settle overnight. No visible residue isformed and all of the mixture is withdrawn and used as the oil phase ina continuous process for forming a HIPE emulsion.

Separate streams of the oil phase (25° C.) and water phase (65°-67° C.)are fed to a dynamic mixing apparatus. Thorough mixing of the combinedstreams in the dynamic mixing apparatus is achieved by means of a pinimpeller. The pin impeller comprises a cylindrical shaft of about 21.6cm in length with a diameter of about 1.9 cm. The shaft holds 4 rows ofpins, 2 rows having 17 pins and 2 rows having 16 pins, each having adiameter of 0.5 cm extending outwardly from the central axis of theshaft to a length of 1.36 cm. The pin impeller is mounted in acylindrical sleeve which forms the dynamic mixing apparatus, and thepins have a clearance of 3 mm from the walls of the cylindrical sleeve.

A minor portion of the effluent exiting the dynamic mixing apparatus iswithdrawn and enters a recirculation zone, as shown in FIG. 1 of U.S.application Ser. No. 08/370,694, filed Jan. 10, 1995, by T. DesMarais.The Waukesha pump in the recirculation zone returns the minor portion tothe entry point of the oil and water phase flow streams to the dynamicmixing zone.

A spiral static mixer is mounted downstream from the dynamic mixingapparatus to provide back pressure in the dynamic mixing apparatus andto provide improved incorporation of components into the HIPE that iseventually formed. The static mixer (TAH Industries Model 100-612) is14.7 inches (37.3 cm) long with a 1.05 inch (2.67 cm) outside diameter.

The combined mixing and recirculation apparatus set-up is filled withoil phase and water phase at a ratio of 4 parts water to 1 part oil. Thedynamic mixing apparatus is vented to allow air to escape while fillingthe apparatus completely. The flow rates during filling are 1.89 g/secoil phase and 7.57 cc/sec water phase. When the apparatus is filledrecirculation is begun at about 9.5 cc/sec recirculation.

Once the apparatus set-up is filled, the water phase flow rate is cut to5.67 cc/sec to reduce the pressure build up while the vent is closed.Agitation is then begun in the dynamic mixer, with the impeller miningat 500 RPM. The flow rate of the water phase is then steadily increasedto a rate of 37.8 cc/sec over a time period of about 1 min., and the oilphase flow rate is reduced to 0.95 g/sec over a time period of about 3min. The recirculation rate is steadily increased to about 39 cc/secduring the latter time period. The Waukesha pump speed is then steadilydecreased to a yield a recirculation rate of about 20 cc/sec, and theimpeller speed is steadily decreased to 400 RPM. The back pressure atthis point is about 1.5 PSI (10 kPa). After some time, the water and oilphase flow rates are reduced by half over a time period of about 50 sec.The impeller RPM is reduced to 200 RPM over about the same time period,and the recirculation rate is reduced to 10 cc/sec. The back pressure atthis point is about 1.2 PSI (8 kPa).

B) Polymerization of HIPE

The HIPE flowing from the static mixer at this point is collected in around polypropylene tub, 17 in. (43 cm) in diameter and 7.5 in (10 cm)high, with a concentric insert made of Celcon plastic. The insert is 5in (12.7 cm) in diameter at its base and 4.75 in (12 cm) in diameter atits top and is 6.75 in (17.1 cm) high. The HIPE-containing tubs are keptin a room maintained at 65° C. for 18 hours to bring aboutpolymerization and form the foam.

C) Foam Washing and Dewatering

The cured HIPE foam is removed from the curing tubs. The foam at thispoint has residual water phase (containing dissolved emulsifiers,electrolyte, initiator residues, and initiator) about 40-50 times(40-50×) the weight of polymerized monomers. The foam is sliced with asharp reciprocating saw blade into sheets which are 0.075 inches (0.19cm) in thickness. These sheets are then subjected to compression in aseries of 2 porous nip rolls equipped with vacuum which gradually reducethe residual water phase content of the foam to about 1 times (1×) theweight of the polymerized material. At this point the sheets are thenresaturated with a 1.5% CaCl₂ solution at 60° C., are squeezed in aseries of 3 porous nip rolls equipped with vacuum to a water phasecontent of about 1×. The CaCl₂ content of the foam is between 1 and 3%.

Table 3 shows data for two foams prepared according to the processesdetailed above, and one foam prepared using different (high shear)processing conditions.

                  TABLE 3                                                         ______________________________________                                                                     Recircu-                                                                             Free                                      Sam- Pour      Mixer   W:O   lation Absorbent                                 ple  Temperature                                                                             RPM     Ratio Rate   Capacity                                                                             RTCD                               ______________________________________                                        A    165° F.                                                                          750     59.8:1                                                                              0      57.2   41.4%                              B.sup.1                                                                            155° F.                                                                          301     40.0:1                                                                              2.5    39.5   21.8%                              C.sup.2                                                                            147° F.                                                                          200     39.6:1                                                                              1.3    48.0   61.3%                              ______________________________________                                              Capacity after      Cell Hole                                                 Desorption          Size Size  50% Desorption                           Sample                                                                              (g/g)*     Shear Rate                                                                             (μm)                                                                            (μm)                                                                             Height (cm)                              ______________________________________                                        A      38%       2767 sec.sup.-1                                                                        NA   11.5  27                                       B.sup.1                                                                             5.8%        540 sec.sup.-1                                                                        133  24.0  19                                       C.sup.2                                                                             3.3%        180 sec.sup.-1                                                                        169  29.3  16                                       ______________________________________                                         .sup.1 : Prepared according to Example 1, above.                              .sup.2 : Prepared according to Example 2, above.                              *: measured from desorption curve as percent capacity at 30 cm (g/g)          compared with free absorbent capacity (g/g).                             

What is claimed is:
 1. An absorbent article especially suitable forabsorbing and retaining aqueous body fluids, said article comprising:I)a backing sheet; and II) an absorbent core associated with said backingsheet such that said absorbent core is positioned between said backingsheet and the fluid discharge region of the wearer of the article, saidabsorbent core comprising a polymeric foam material which is capable ofacquiring and distributing aqueous fluids, said polymeric foam materialcomprising a hydrophilic, flexible, nonionic polymeric foam structure ofinterconnected open cells, which foam structure has:A) a capacity afterdesorption of 30 cm of less than about 10% of the foam's free absorbentcapacity; B) a capillary absorption pressure of from about 3 to about 20cm; C) a capillary desorption pressure of from about 8 to about 25 cm;D) a resistance to compression deflection of from about 5 to about 85%when measured under a confining pressure of 0.74 psi; and E) a freeabsorbent capacity of from about 12 to about 125 g/g.
 2. The absorbentarticle of claim 1 wherein said absorbent core comprises: (1) afluid-handling layer comprising the foam material positioned in saidfluid discharge region; and (2) a fluid storage/redistribution layer influid communication with said fluid-handling layer.
 3. The absorbentarticle of claim 2 wherein said backing sheet is liquid impervious andwhich additionally comprises a liquid pervious topsheet joined with saidbacking sheet, said absorbent core being positioned between saidtopsheet and said backing sheet.
 4. The absorbent article of claim 3which is a diaper.
 5. A diaper useful for absorbing aqueous body fluidsdischarged by an incontinent individual, said diaper articlecomprising:I) a relatively liquid-impervious backing sheet; II) arelatively liquid-pervious topsheet; III) an absorbent core positionedbetween said backing sheet and said topsheet, said absorbent corecomprising a polymeric foam material which is capable of acquiring anddistributing aqueous fluids, said polymeric foam material comprising ahydrophilic, flexible, nonionic polymeric foam structure ofinterconnected open cells, which foam structure has:A) a capacity afterdesorption at 30 cm of less than about 10% of the foam's flee absorbentcapacity; B) a capillary absorption pressure of from about 3 to about 20cm; C) a capillary desorption pressure of from about 8 to about 25 cm;D) a resistance to compression deflection of from about 5 to about 85%when measured under a confining pressure of 0.74 psi; E) a freeabsorbent capacity of from about 12 to about 125 g/g; and F) a recoveryfrom wet compression of at least about 60%.
 6. The diaper of claim 5wherein said absorbent core comprises: (1) a fluid-handling componentcomprising the foam material positioned in the fluid discharge region ofthe wearer of the diaper article; and (2) a fluid storage component influid communication with said fluid-handling component.
 7. The diaper ofclaim 6 wherein said fluid storage component comprises absorbent gellingmaterial.
 8. The diaper of claim 7 wherein said absorbent gellingmaterial is laminated between two tissue layers.
 9. The diaper of claim7 wherein the foam material has about 2% or less residual hydratablesalts.
 10. The diaper of claim 6 wherein said fluid storage componentcomprises an absorbent foam.
 11. The diaper of claim 6 wherein said foammaterial is a polymerized water-in-oil emulsion having:1) an oil phasecomprising:a) from about 85 to about 98% by weight of a monomercomponent capable of forming a copolymer having a Tg value of belowabout 35° C. or lower, said monomer component comprising:i) from about30 to about 80% by weight of a substantially water-insoluble,monofunctional monomer capable of forming a polymer having a Tg of about25° C. or less; ii) from about 5 to about 40% by weight of asubstantially water-insoluble, monofunctional comonomer capable ofimparting toughness about is equivalent to that provided by styrene;iii) from about 5 to about 25% by weight of a first substantiallywater-insoluble, polyfunctional crosslinking agent selected from thegroup consisting of divinyl benzenes, trivinyl benzenes, divinyltoluenes, divinylxylenes, divinylnaphthalenes, divinylalkylbenzenes,divinylphenanthrenes, divinylbiphenyls, divinyldiphenylmethanes,divinylbenzyls, divinylphenylethers, divinyldiphenylsulfides,divinylfurans, divinylsulfide, divinylsulfone, and mixtures thereof; andiv) from 0 to about 15% by weight of a second substantiallywater-insoluble, polyfunctional crosslinking agent selected from thegroup consisting of polyfunctional acrylates, methacrylates,acrylamides, methacrylamides, and mixtures thereof; b) from about 2 toabout 15% by weight of an emulsifier component which is soluble in theoil phase and which is suitable for forming a stable water-in-oilemulsion; and 2) a water phase comprising from about 0.2 to about 20% byweight of a water-soluble electrolyte; 3) a volume to weight ratio ofwater phase to oil phase in the range of from about 25:1 to about 90:1.12. The diaper of claim 11 wherein:1) the oil phase comprises:a) fromabout 90 to about 97% by weight of a monomer component capable offorming a copolymer having a Tg value from about 15° to about 30° C.,said monomer component comprising:i) from about 50 to about 65% byweight monomer selected from the group consisting of C₄ -C₁₄ alkylacrylates, aryl acrylates, C₆ -C₁₆ alkyl methacrylates, C₄ -C₁₂ alkylstyrenes and mixtures thereof; ii) from about 15 to about 25% by weightcomonomer selected from the group consisting of styrene, ethyl styreneand mixtures thereof; iii) from about 12 to about 20% by weightdivinylbenzene; and iv) from 0 to about 13% by weight of said secondcrosslinking agent selected from the group consisting of 1,4-butanedioldimethacrylate, ethylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, and mixtures thereof; b) from about 3 to about 10% by weightof said emulsifier component; 2) the water phase comprises from about 1to about 10% calcium chloride; 3) the volume to weight ratio of waterphase to oil phase is in the range of from about 30:1 to about 65:1.